OPINION OF THE COURT
ALITO, Circuit Judge:*
This is an appeal in a copyright case. Southco, Inc. alleges that Kanebridge Corp. violated its copyright by referring to the serial numbers that Southco assigned to certain parts that it manufactures. The District Court issued a preliminary injunc[278] tion forbidding Kanebridge from making such references, but a panel of this Court reversed, holding that Southco was unlikely to succeed on the merits because the serial numbers lacked sufficient originality to be copyrighted. On remand, the District Court granted Kanebridge’s motion for summary judgment on the copyright claim, but a different panel of our Court reversed, holding that an affidavit submitted by Southco in opposition to Kane-bridge’s summary judgment motion was sufficient to demonstrate that the numbers reflected considerable creativity. We now hold that the numbers are not protected by copyright, and we therefore affirm the order granting summary judgment in favor of Kanebridge.
I.
Southco manufactures a variety of products, including rivets, latches, handles, and “captive fasteners” that are used to fasten two panels together. A “captive” fastener is one whose components are retained in the outer panel when the two panels are detached. “Captive screws” are a type of captive fastener. Each captive screw consists of a “knob” (the component that surrounds the screw head), the screw itself, and a “ferrule” (a component that houses the screw). The captive screw is mounted in the outer panel by means of the ferrule. The other panel contains an internally threaded insert that receives the screw. Captive screws differ among themselves with respect to a few characteristics, such as composition, screw length, screw diameter, thread size, and finish.
To assist its employees and customers in identifying and distinguishing among its products, Southco developed a numbering system under which each particular digit or group of digits signifies a relevant characteristic of the product. Southco has referred to one of the numbers at issue in this case, part number 47-10-202-10, to show how the system works. The first two digits (“47”) show that the part falls within a class of captive screws. Other digits indicate characteristics such as thread size (“632”), composition of the screw (aluminum), and finish of the knob (“knurled”). See Southco, Inc. v. Kanebridge Corp., 258 F.3d 148, 149 n. 2 (3d Cir.2001) (“Southco I”) (quoting Southco’s brief).
A person who understands the Southco system can use it in two ways. First, the person can readily determine from a product number the characteristics of the product designated — for example, the type of product (say, a screw), the type of screw, and all of the characteristics that a user might need to know. Second, working in reverse, a person who knows the characteristics of the product needed for a particular job can determine the number of the product with the desired characteristics. Southco includes its product numbers in handbooks that it publishes each year, and Southco has secured copyright registrations for several of its handbooks.
According to Southco, its part numbers' play a significant role in the subcontracting of work on computers and telecommunications equipment. Computer and telecommunications equipment manufacturers often use “subassemblies” supplied by subcontractors, and manufacturers often use Southco part numbers to specify the captive fasteners to be used in these subas-semblies. However, manufacturers sometimes permit subcontractors to substitute equivalent fasteners manufactured by companies other than Southco, and this gives the subcontractors an incentive to substitute cheaper fasteners made by Southco’s competitors. See Joint Appendix (“A”) 18-19.
Matdan America (“Matdan”) is a South-co competitor that manufactures panel fas[279] teners. Kanebridge, known as Matdan’s “master distributor,” sells Matdan fasteners to other distributors, often at prices lower than Southco’s. In order to demonstrate that its fasteners have the same characteristics as Southco’s but are sold at lower prices, Kanebridge began to use Southco’s part numbers in comparison charts that were included in advertisements and other literature provided to customers. These charts display Kane-bridge’s and Southco’s numbers for equivalent fasteners in adjacent columns, making it clear that the two companies’ parts are interchangeable. According to Kane-bridge, the “ability to cross-reference Southco panel fasteners in an honest, accurate and comparative manner” is necessary to make competition viable. Kane-bridge’s Southco I Brief at 7. Without this ability, Kanebridge insists, customers would lose the opportunity to obtain lower-cost alternative fasteners. Id.
Southco commenced this action against Kanebridge, asserting a claim for copyright infringement under 17 U.S.C. §§ 501-05, 509, as well as Lanham Act claims for false advertising (15 U.S.C. § 1125(a)), trademark infringement (15 U.S.C. § 1114(1)), and unfair competition (15 U.S.C. § 1125(a)), and claims for common law trademark infringement and trademark dilution. In support of its copyright infringement claim, Southco alleged that Kanebridge had copied 51 part numbers for Southco’s “Class 47 captive screw fasteners.” A23. Examples of the numbers that Southco claimed are protected by copyright are the following:
47-10-202-10
47-11-502-10
47-10-502-50
47-12-502-50
47-62-501-20
A24. Southco alleged that Kanebridge had used these copyrighted numbers in “advertising, product brochures, catalogs, reference guides, packaging and/or price lists.” Id.
The parties agreed to a temporary restraining order containing various restrictions on Kanebridge’s use of Southco’s part numbers, but when the parties failed to agree on the scope of a preliminary injunction, Southco moved for a preliminary injunction preventing Kanebridge from making any reference to Southco’s numbers. The District Court granted the motion, concluding, among other things, that Southco’s “numbering system is copyrighted.” Southco, Inc. v. Kanebridge Corp., No. 99-4337, 2000 WL 21257, at *1 (E.D.Pa.2000) (emphasis added). The Court wrote:
The Numbering System, with its unique, non-intuitive and highly complex attributes, easily satisfies the standard for originality. It was created out of nothing, and has developed to some use as an industry standard.... It is expandable as new products are developed, and is of use to Southco employees and customers.
Id. at *3 (emphasis added).
On appeal, a panel of this Court reversed. Southco I, 258 F.3d at 148. The Southco I panel began by noting that, “[f]or purposes of copyright law, ... Southco’s numbering system and the actual numbers produced by the system are two very different works” and that South-co’s claim was based exclusively on the actual numbers and not on the system. Id. at 151-52 (footnote omitted). The panel wrote that Southco had “unquestionably devoted time, effort, and thought to the creation of the numbering system” but that Southco’s system made it “impossible for the numbers themselves to be original.” Id. at 153 (emphasis omitted). Focusing on the use of the Southco system to [280] assign a number to a product in an existing product line,1 the Southco I panel wrote:
The part has certain relevant characteristics, and the numbering system specifies certain numbers for each of those characteristics. As a result, there is only one possible part number for any new panel fastener that Southco creates. This number results from the mechanical application of the system, not creative thought. If Southco were to develop a new fastener and for some reason decide to exercise creativity when assigning it a number, the resulting part number would fail to accomplish its purpose. Regardless of how small the change is, customers could not effectively identify the relevant characteristics of the panel fastener by simply looking at its part number.
Id. at 153. The Southco I panel thus concluded that “the creative spark is utterly lacking in Southco’s part numbers and that these numbers are examples of works that fall short of the minimal level of creativity required for copyright protection.” Id. at 152.
On remand, Kanebridge moved for summary judgment. In response, Southco submitted the affidavit of Robert H. Bisb-ing, a retired Southco engineer who had designed numerous fasteners for Southco and had assigned them product numbers. A262-63. Bisbing explained how he had assigned product numbers to a new class of enclosed retractable captive screws that included 405 variations. A264-71. Bisb-ing recounted that “it had long been Southco’s practice to create a system of numbers for each class of its products,” including classes of drive rivets, latches, pulls, and handles, as well as fasteners. A264. He stated that the relevant characteristics of this new class differed from those of previous classes and that he therefore adapted the system to apply to the new line. A267. Bisbing elaborated:
Although there were a variety of South-co numbering systems in existence in 1971 when I created the enclosed retractable captive screw, including for several classes of captive screws, none of them could be used for the new product. Each of the systems of part numbers that had been created for existing products identified particular values pertinent to those products which would not be useful in the new fasteners.
Id. Bisbing therefore identified the characteristics to be designated in the product numbers for the new class and assigned numbers to represent variations within each characteristic. A267-71. For example, he decided to use the fifth digit in each number to show thread size. A269.
The District Court expressed continuing disagreement with the decision in Southco I but nevertheless granted Kanebridge’s motion for summary judgment on the copyright infringement claim, finding that Bisbing’s affidavit provided no new material evidence. A305-06. The District Court further noted that Bisbing’s affidavit concerned the creation of the system, not the numbers, and that, under Southco I, the creativity of the numbers was the only relevant issue. A314. The parties settled the remaining claims, and Southco appealed the order granting summary judgment on the copyright infringement claim.
On appeal, a panel of our Court reversed. Southco, Inc. v. Kanebridge Corp., 324 F.3d 190, 193 (3d Cir.2003) [281] (“Southco II”). The Southco II panel stated that “the Southco I panel’s conclusion regarding originality involved, at least in part, factual determinations regarding the process by which Southco develops numbers” and that the Bisbing declaration suggested that the prior panel’s “conclusions regarding the relationship between the numbering system and the product numbers ha[d] no basis in fact.” Id. at 196. According to the Southco II panel, the earlier panel had labored under the false impression that “the product numbers are mechanically dictated by a preexisting numbering system.” Id. at 196. The Southco II panel concluded that Bisb-ing’s declaration “call[ed] into doubt the Southco I panel’s factual conclusions about the process by which Southco assigns numbers to new fasteners.” Id. at 197. The Southco II panel continued:
Bisbing’s declaration does not indicate that the inventor of a new fastener determined the physical characteristics of the new fasteners and then consulted Southco’s numbering system to mechanically assign a product number that described those physical characteristics. To the contrary, the Bisbing declaration states that Bisbing exercised creativity and choice in determining the values to be reflected in the numbers.
Id. at 197. The Southco II panel therefore reversed the decision of the District Court and remanded “for consideration of the Bisbing declaration.” Id. We subsequently voted to rehear this case en banc and vacated the panel opinion.
II.
We hold that the Southco part numbers are not copyrightable. Two different lines of reasoning lead us to this conclusion.
A.
First, as we held in Southco I, the Southco numbers are not “original.” Under Article I, section 8 of the Constitution, Congress has the power “to seeur[e] for limited Times to Authors ... the exclusive Right to their respective Writings.” As used in this provision, the terms “Authors” and “Writings” “presuppose a degree of originality,” and therefore “Originality is a constitutional requirement.” Feist Publications. Inc. v. Rural Tel. Serv. Co., 499 U.S. 340, 346, 111 S.Ct. 1282, 113 L.Ed.2d 358 (1991). Accordingly, Congress has provided copyright protection for “original works of authorship fixed in any tangible medium of expression.” 17 U.S.C. § 102(a) (emphasis added). In order to satisfy the “original works” requirement, a work must be original in the sense that it was not copied from another’s work and in the sense that it shows creativity (“the creativity requirement”). Feist, 499 U.S. at 361-363, 111 S.Ct. 1282. Although the creativity requirement is not “stringent,” there is “a narrow category of works in which the creative spark is utterly lacking or so trivial as to be virtually nonexistent.” Id. at 358-59, 111 S.Ct. 1282.
Feist illustrates the meaning of the creativity requirement. There, the Rural Telephone Service Company, a provider of local telephone service, published a typical phone book that contained “white pages” listing in alphabetical order the names, towns, and telephone numbers of its subscribers. Feist, 499 U.S. at 342, 111 S.Ct. 1282. Rural held a valid copyright in its book as a whole because it contained some original material in the forward and in the yellow pages, and Rural claimed that Feist, which published its own phone book, had infringed Rural’s copyright by copying the names, numbers, and towns of the subscribers listed in Rural’s white pages. Id. at 344, 361, 111 S.Ct. 1282. Rural “essentially concede[d]” that the names, [282] addresses, and numbers were uneopyrightable facts, but Rural argued that its selection, coordination, and arrangement of these facts reflected sufficient originality to merit copyright protection. Id. at 361-62, 111 S.Ct. 1282.
The Supreme Court rejected this argument because Rural published “a garden-variety white pages directory, devoid of even the slightest trace of creativity.” Feist, 499 U.S. at 362, 111 S.Ct. 1282. The Court observed that “Rural’s selection of listings could not be more obvious: It publishes the most basic information — name, town, and telephone number — about each person who applies to it for telephone service.” Id. “This is ‘selection’ of a sort,” the Court stated, “but it lacks the modicum of creativity necessary to transform mere selection into copyrightable expression.” Id. The Court also saw little creativity in Rural’s “coordination and arrangement of facts,” noting that Rural had simply listed its subscribers in alphabetical order, “an age-old practice, firmly rooted in tradition and so commonplace that it has come to be expected as a matter of course.” Id. at 363, 111 S.Ct. 1282.
B.
In this case, the Southco product numbers are not “original” because each number is rigidly dictated by the rules of the Southco system. Because ideas may not be copyrighted, Southco does not assert any claim of copyright in its numbering system, but instead focuses on the part numbers themselves. The numbers, however, do not reflect any creativity.
To be sure, before any Southco product could be numbered, Southco had to create the numbering system applicable to products in that line. It had to identify the relevant characteristics2 of the products in the class (that is, the characteristics that would interest prospective purchasers); it had to assign one or more digits to express each characteristic; and it had to assign a number or other symbol to represent each of the relevant values3 of each characteristic. For example, Southco might decide that, for a class of screws, composition was a relevant characteristic; it might assign the eighth digit to indicate composition; and it might use the number 1 to indicate aluminum, 2 to indicate steel, and so on.
Once these decisions were made, the system was in place, and all of the products in the class could be numbered without the slightest element of creativity. Indeed, if any creativity were allowed to creep into the numbering process, the system would be defeated. Suppose, for example, that the person given the task of actually numbering the products in the class in accordance with the applicable rules of the system decided that it would be more fitting to indicate aluminum composition with the number 13 (its number in the periodic chart) rather than the number 1. Customers who wished to purchase aluminum screws but were unaware of this variation would be befuddled. In short, an essential attribute of the numbering process and the resulting numbers is an utter absence of creativity. We thus reaffirm what we said in Southco I: the number assigned to each Southco product “results from the mechanical application of the system, not creative thought.” 258 F.3d at 153. As a leading treatise states, “basic copyright principles” lead to the conclusion that copyright protection should not be extended to part numbers that represent “an inevitable sequence dictated by the logic of the parts.system.” 1 William F. [283] PatRY, Copyright Law and Practice 46 (2d ed.2004) (forthcoming). See also 1 John W. Hazard, Jr., Copyright Law in Business and Practice § 2.58 at 2-79 to 2-80 (2002) (Southco I “persuasive” in distinguishing between numbers assigned pursuant to process that requires creativity and numbers assigned under system that leaves no room for creativity).
Nothing in the Bisbing declaration undermines this analysis. The Bisbing declaration points out that before the parts in a particular product class (i.e., a group of products having the same relevant characteristics) can be numbered, a person must identify the product characteristics to be reflected in the numbers and must devise the code to be used to express those characteristics. Southco had to undertake this process when it first devised its system, and it must engage in a similar process whenever it undertakes to number a new product class with relevant characteristics that are different from the products in the existing classes.
A few examples illustrate the point. Suppose that Southco, after originally devising its system for the purpose of numbering fasteners, decided to market and number a very different product, say, handles. In assigning numbers to its handles, Southco obviously could not simply use the system devised for its fasteners because door handles and fasteners have very different characteristics. For instance, an important characteristic of screws is the type of recess on the head, e.g., regular or Phillips. This is obviously not a relevant characteristic of handles.
To take another example, suppose that Southco, after previously numbering captive screws, decided to number a related but nevertheless different product, say, captive fasteners that can be tightened by hand. Again, because such fasteners do not have the same characteristics as captive screws (for instance, like handles, they also lack recesses for a regular or Phillips screwdriver), the system would have to be modified. All of this shows that a certain degree of thought goes into the development of the system for numbering each product line. But once the rules of the system applicable to the particular product class are set, the numbers themselves are generated by a mechanical application of the rules and do not reflect even a spark of creativity.
In arguing that its product numbers satisfy the creativity requirement, Southco relies heavily on Burrow-Giles Lithographic Co. v. Sarony, 111 U.S. 53, 4 S.Ct. 279, 28 L.Ed. 349 (1884), which concerned the copyrightability of a photograph of Oscar Wilde. A statute in force at the time provided expressly that a photograph could be copyrighted, and therefore the Court turned to the question whether the statute was authorized by the Constitution. Id. at 55-56, 4 S.Ct. 279. Based on early statutes and long settled practice, the Court concluded that maps, engravings, and etchings could be copyrighted, and the Court added that “[t]he only reason why photographs were not included” in one of the early statutes was “probably” because “photography, as an art, was then unknown.” Id. at 58, 4 S.Ct. 279.
The Court then addressed the defendant’s argument that no photograph could be copyrighted because a photograph does not “embody the intellectual conception of its author” but is instead “the mere mechanical reproduction of the physical features or outlines of some object animate or inanimate, and involves no originality of thought or any novelty in the intellectual operation connected with its visible reproduction in shape of a picture.” Burrow-Giles, 111 U.S. at 58-59, 4 S.Ct. 279. The Court stated that “[t]his may be true in regard to the ordinary production of a [284] photograph, and, further, that in such case a copyright is no protection.” Id. But the Court found it unnecessary to consider that question because it held that the particular photograph at issue was “an original work of art, the product of plaintiffs intellectual invention.” Id. at 60, 4 S.Ct. 279. In reaching this conclusion, the Court noted a set of findings to the effect that the photograph reflected the plaintiffs “own original mental conception, to which he gave visible form” by posing Oscar Wilde, “suggesting and evoking the desired expression,” selecting and arranging “the costume, draperies, and other various accessories,” and “arranging and disposing the light and shade.” Id. (internal quotations omitted).
Southco argues that “the present case is similar to Burrow-Giles.” Southco Br. at 33. Southco likens the creation of its numbering system to creative work done by the photographer prior to snapping the photograph of Oscar Wilde, and Southco suggests that the next step in its case (i.e., the numbering of the Southco parts) was no less mechanical than the chemical processes that produced the photograph. Id. We see no merit in this analogy.
Southco’s flawed analogy requires acceptance of the Burrow-Giles defendant’s simplistic description of photography. The Burrow-Giles defendant basically contended that a photographer does not create a picture (as a painter or engraver does) but simply uses a machine to capture a bit of reality that existed at a particular place and time. In other words, while a painting or engraving is an expression of ideas in the artist’s mind, a photograph is a bit of objective reality. Where, as in the case of the Oscar Wilde portrait, the photographer poses the subject, the photographer may exercise creativity in arranging the bit of reality to be captured by the photo, but the photograph itself is not expression.
If this view of photography were correct, photography could be analogized to the operation of the Southco numbering system, which objectively captures a few functional characteristics of products like screws. But the Burrow-Giles defendant’s description of photography is plainly inaccurate' — as the Burrow-Giles decision recognized, at least with respect to the not “ordinary” photo that was before the Court.
The Southco numbers are purely functional; the portrait of Oscar Wilde, whatever its artistic merit, was indisputably a work of art. The Southco numbers convey information about a few objective characteristics of mundane products' — -for example, that a particular screw is one-eighth of an inch in length. A photographic portrait, by contrast, does not simply convey information about a few objective characteristics of the subject but may also convey more complex and indeterminate ideas. The Southco numbers are produced mechanically using a system with fixed rules. No photographic portrait is produced in a comparable way. While a portrait photographer may use conventional principles of photographic composition, those principles are not at all like the fixed rules of the Southco system. Accordingly, there is no real analogy between Southco’s numbers and the Oscar Wilde photograph in Burrow-Giles.
There is also no merit to the analogy suggested at oral argument between the Southco numbers and a painting that an artist creates by causing paint to drop onto a canvass. An aleatoric painting (or other work of aleatoric art) does not result from the rigid application of a system of pre-set rules. On the contrary, the randomness that is employed expresses the artist’s “mental conception.” Burrow-Giles, 111 U.S. at 60, 4 S.Ct. 279. See 1 William F. Patry, supra, at 44.
[285] In sum, we hold that the Southco part numbers are not protected by copyright because they are mechanically produced by the inflexible rules of the Southco system.4
III.
The Southco part numbers are also excluded from copyright protection because they are analogous to short phrases or the titles of works. Since at least 1899, it has been the practice of the Copyright Office to deny registration to “words and phrases.” 1 W. Patry, Copyright Law AND Practice 333 n. 89 (1994). In a 1958 circular, the Copyright Office stated:
To be entitled to copyright protection, a work must contain something capable of being copyrighted — that is, an appreciable amount of original text or pictorial material.... Brand names, trade names, slogans, and other short phrases or expressions cannot be copyrighted, even if they are distinctively arranged or printed.
Circular No. 46, Copyright In Commercial Prints and Labels (1958) (emphasis added). This circular went on to suggest that protection for short phrases used in connection with commercial products is more appropriately addressed under federal trademark law and laws relating to unfair competition. Id.
Shortly after publishing this circular, the Copyright Office issued a regulation providing that words and short phrases such as names and titles may not be copyrighted. See 24 Fed.Reg. 4956 (June 18, 1959). The current version of this regulation now provides in relevant part:
The following are examples of works not subject to copyright and applications for registration of such works cannot be entertained:
(a) Words and short phrases such as names, titles, and slogans; familiar symbols or designs; mere variations of typo[286] graphic ornamentation, lettering or coloring; mere listing of ingredients or contents....
37 C.F.R. § 202.1 (2004) (emphasis added).
In Kitchens of Sara Lee, Inc. v. Nifty Foods Corp., 266 F.2d 541, 544 (2d Cir.1959), the Second Circuit endorsed this principle and termed the above regulation a “fair summary of the law.” Accord, e.g., Alberto-Culver Co. v. Andrea Dumon, Inc., 466 F.2d 705, 711 (7th Cir.1972); CMM Cable Rep, Inc. v. Ocean Coast Props., Inc., 97 F.3d 1504, 1519-20 (1st Cir.1996); Murray Hill Publ’ns, Inc. v. ABC Communications, Inc., 264 F.3d 622, 632-33 (6th Cir.2001).
Relying on the short phrases regulation, the government tells us, “the Register of Copyrights routinely determines that a part number does not ‘constitute[ ] copyrightable subject matter’ ” under 17 U.S.C. § 410. U.S. Amicus Br. at 13. The government also calls to our attention letters from the Examining Division of the Copyright Office that illustrate this practice, and the government notes that Congress has not disturbed “the Copyright Office’s long-standing practice against registering short phrases, despite repeated and extensive revisions of the copyright code.” Id. at 17.
The government suggests that this practice serves at least two purposes. First, the government notes that “[a] short phrase such as a part number typically lacks any creativity whatsoever.” U.S. Amicus Br. at 11. Second, the government suggests that extending copyright protection to part numbers would unduly interfere with the legitimate use of the numbers in question. Id. at 15. Because the owner of a copyright “has the exclusive rights” “to reproduce the copyrighted work,” 17 U.S.C. § 106, if a part number (say, 471020210, to take the example discussed above) were copyrighted, any use of the number would potentially infringe the copyright. Moreover, if Southco’s nine-digit numbers are protected, would there be a principled basis for denying protection to a number with, say, seven or five digits? Could a company or person thereby obtain the exclusive right to use the number 4,710,202 or 47,102? In light of the huge number of part and product numbers (and other analogous numbers) that now exist, this prospect gives reason for concern. Although the fair use defense would presumably protect the use of such numbers in most situations, fair use is an affirmative defense and may impose an undue burden.
We believe that the Copyright Office’s longstanding practice of denying registration to short phrases merits deference.5 See DeSylva v. Ballentine, 351 U.S. 570, 577-78, 76 S.Ct. 974, 100 L.Ed. 1415 (1956); Morris v. Business Concepts, 283 F.3d 502, 505-06 (2d Cir.2002) (Skidmore deference); Marascalco v. Fantasy, Inc., 953 F.2d 469, 473 (9th Cir.1991); Cablevision Systems Dev. Co. v. Motion Picture Ass’n of Am., Inc., 836 F.2d 599, 608-10 & n. 16 (D.C.Cir.1988) (Chevron deference).6 [287] We accept the Copyright Office position and believe that it logically extends to part numbers.
IY.
For the reasons set out above, we hold that the Southco part numbers are not entitled to copyright protection. We therefore affirm the order of the District Court granting summary judgment in favor of Kanebridge on Southco’s copyright infringement claim.7
There is also no tension between our decision in this case and Whelan Assocs., Inc. v. Jaslow Dental Laboratory, Inc., 797 F.2d 1222, 1236 (3d Cir.1986). In Whelan Assocs., we held that the non-literal structure of a computer program represented expression, rather than an idea, and was entitled to copyright protection. The dissent argues that the test that we used in Whelan Assocs. to distinguish between idea and expression "strongly supports the position that Southco's numbering of products should fall on the 'expression' side of the line,” Dissent at 10-11, but this argument is beside the point. No one has ever suggested that the Southco part numbers fall on the "idea” side of the line. The relevant question is not whether the numbers represent an idea, as opposed to the expression of the idea, but whether the numbers possess the requisite spark of creativity needed for copyright protection.
The dissent advances an argument that is very different from the position that Southco has taken throughout this litigation. The dissent contends that the expression at issue here consists of both "the particular numbers” at issue and "the numbering rules” that produced those numbers. Dissent at 3. But Southco has never claimed that its numbering "rules” or "system” is copyrightable. In its brief in Southco I, counsel for Southco wrote: "There is no monopoly on the system; anyone is free to use it ... with impunity. It is only the particular expression that Southco seeks to protect — the precise nine digits which express the idea in each part number.” Appellee's Br. at 14, Southco, Inc. v. Kanebridge Corp., No. 00-1102. Counsel reiterated this point at oral argument, stating: "The system is not copyrightable. And, indeed, it's perfectly open to the defendants to use the system. They’re free to use it. The only thing that we’re protecting here are the expressions.”