SWYGERT, Circuit Judge.
This case concerns the validity of U.S. Patent No. 3,991,831 (Foster) covering the invention of a harrow used in cultivating ground for agriculture purposes. Lee Blacksmith, Inc. (“Blacksmith”), the owner of the patent brought this action against Lindsay Brothers, Inc. (“Lindsay”) for inducing infringement. Lindsay counterclaimed alleging invalidity of the patent under 35 U.S.C. § 102(b) (“on sale” bar) and 35 U.S.C. § 103 (obviousness). The district court held the Foster patent valid and found Lindsay liable for infringement. We reverse. Without reaching the section 102(b) issue, we hold the Foster patent invalid for obviousness under section 103.
I
The Foster patent has one claim. The invention described in this claim is the combination of a harrow and a post-plowing cultivating implement, such as a disc harrow (parent implement). The harrow is attached rigidly to the frame of the parent implement by one or more support beams. As illustrated by Diagrams 1 and 2 below, an arm is pivotally connected to and depends on each beam for movement toward and away from the parent implement. An elongated rail is attached to the lower end of the arms and extends laterally therefrom so that it is parallel to the rearmost row of the soil-working parts of the parent implement. A series of spaced, parallel metal rods (teeth), attached to the rail, slant downwardly and toward one end of the rail and form an acute angle with the rail. The teeth are spaced six inches apart and the length of the teeth is such that the lower end of each tooth projects under the upper end of an adjacent tooth. An adjustable spring connected between the frame of the parent implement and the pivotal arms of the harrow urges the teeth into the ground, with the teeth usually being tilted back at an angle of about forty-five degrees.
The alleged unique feature of the Foster patent is the acute angle of the tooth to the rail and the spacing of the teeth so that the lower end of each tooth projects under the upper end of an adjacent tooth. In its findings of fact the district court stated that this construction of a harrow produces a synergistic effect in that “all, or effectively all of the earth traversed by the harrow ... is engaged by a tooth
II
When a patent’s validity is challenged, the patent is presumed to be valid and the burden of establishing invalidity rests on the party asserting it. 35 U.S.C. § 282. The presumption of validity, however, does not exist against prior art which [343]*343was not considered by the Patent Office when the patent was issued. Allen Group v. Nu-Star, Inc., 575 F.2d 146 (7th Cir. 1974) (per curiam). Lindsay produced before the district court two patents not cited: Sigmund U.S. Patent No. 3,574,320 and Hotchkiss Patent No. 3,090,448, and, relying on Allen Group, supra, contended that the presumption of validity should not apply to the patent in suit. But the district court found that the prior art relied upon by Lindsay was the same or no better than that considered by the patent examiner in issuing the patent.1
The district court’s findings will not be set aside unless clearly erroneous.2 The presumption of validity continues when the patent examiner has considered “equivalent references” to the prior art relied upon by the party claiming invalidity. Laser Alignment, Inc. v. Woodruff & Sons, Inc., 491 F.2d 866, 871 (7th Cir. 1974). Thus for the purposes of determining obviousness on this appeal, the Foster patent will be presumed valid.
35 U.S.C. § 103 provides:
A patent may not be obtained . if the differences between the subject matter sought to be patented and the prior art are such that the subject matter as a whole would have been obvious at the time the invention was made to a person having ordinary skill in the art to which said subject matter pertains. Patentability shall not be negatived by the manner in which the invention was made.
In Graham v. John Deere Co., 383 U.S. 1, 86 S.Ct. 684, 15 L.Ed.2d 545 (1966), the Supreme Court outlined the proper analysis to be followed when obviousness is at issue.
While the ultimate question of patent validity is one of law [citation omitted], the § 103 condition, which is but one of three conditions, each of which must be satisfied, lends itself to several basic factual inquiries. Under § 103, the scope and content of the prior art are to be determined; differences between the pri- or art and the claims at issue are to be ascertained; and the level of ordinary skill in the pertinent art resolved.
Id. at 17, 86 S.Ct. at 694. “[T]he Graham analysis [is] the exclusive means by which to measure nonobviousness under section 103.” Republic Industries, Inc. v. Schlage Lock Co., 592 F.2d 963 (7th Cir. 1979).3
In its findings the district court stated that Lindsay did not offer “evidence of any kind on the level of ordinary skill at any time in any art of farm earth-working equipment.” The district court then concluded that because Lindsay had failed to establish evidence on one of the factual inquiries as required by Graham, the defendant had not overcome the presumption of validity. The district court’s conclusion would be correct if Graham required that in every case the level of skill in the pertinent [344]*344art must be resolved. This reading of Graham is overbroad.
The district court incorrectly relied on Universal Athletic Sales Co. v. American Gym, 546 F.2d 530 (3d Cir. 1976), in reaching its conclusion. In Universal the Third Circuit held that a district court could not invalidate a patent when the record was deficient with respect to evidence on the level of skill. But this rule is not without exception.
The correct rule was formulated by this court in Research Corp. v. Nasco Industries, Inc., 501 F.2d 358 (7th Cir.), cert. denied, 419 U.S. 1096, 95 S.Ct. 689, 42 L.Ed.2d 688 (1974). In Research Corp. a patent was held obvious on a motion for summary judgment. No evidence had been heard on the level of skill in the pertinent art. The court stated:
Here there was no factual dispute concerning the state of the prior art, as recorded in the Taylor article.
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SWYGERT, Circuit Judge.
This case concerns the validity of U.S. Patent No. 3,991,831 (Foster) covering the invention of a harrow used in cultivating ground for agriculture purposes. Lee Blacksmith, Inc. (“Blacksmith”), the owner of the patent brought this action against Lindsay Brothers, Inc. (“Lindsay”) for inducing infringement. Lindsay counterclaimed alleging invalidity of the patent under 35 U.S.C. § 102(b) (“on sale” bar) and 35 U.S.C. § 103 (obviousness). The district court held the Foster patent valid and found Lindsay liable for infringement. We reverse. Without reaching the section 102(b) issue, we hold the Foster patent invalid for obviousness under section 103.
I
The Foster patent has one claim. The invention described in this claim is the combination of a harrow and a post-plowing cultivating implement, such as a disc harrow (parent implement). The harrow is attached rigidly to the frame of the parent implement by one or more support beams. As illustrated by Diagrams 1 and 2 below, an arm is pivotally connected to and depends on each beam for movement toward and away from the parent implement. An elongated rail is attached to the lower end of the arms and extends laterally therefrom so that it is parallel to the rearmost row of the soil-working parts of the parent implement. A series of spaced, parallel metal rods (teeth), attached to the rail, slant downwardly and toward one end of the rail and form an acute angle with the rail. The teeth are spaced six inches apart and the length of the teeth is such that the lower end of each tooth projects under the upper end of an adjacent tooth. An adjustable spring connected between the frame of the parent implement and the pivotal arms of the harrow urges the teeth into the ground, with the teeth usually being tilted back at an angle of about forty-five degrees.
The alleged unique feature of the Foster patent is the acute angle of the tooth to the rail and the spacing of the teeth so that the lower end of each tooth projects under the upper end of an adjacent tooth. In its findings of fact the district court stated that this construction of a harrow produces a synergistic effect in that “all, or effectively all of the earth traversed by the harrow ... is engaged by a tooth
II
When a patent’s validity is challenged, the patent is presumed to be valid and the burden of establishing invalidity rests on the party asserting it. 35 U.S.C. § 282. The presumption of validity, however, does not exist against prior art which [343]*343was not considered by the Patent Office when the patent was issued. Allen Group v. Nu-Star, Inc., 575 F.2d 146 (7th Cir. 1974) (per curiam). Lindsay produced before the district court two patents not cited: Sigmund U.S. Patent No. 3,574,320 and Hotchkiss Patent No. 3,090,448, and, relying on Allen Group, supra, contended that the presumption of validity should not apply to the patent in suit. But the district court found that the prior art relied upon by Lindsay was the same or no better than that considered by the patent examiner in issuing the patent.1
The district court’s findings will not be set aside unless clearly erroneous.2 The presumption of validity continues when the patent examiner has considered “equivalent references” to the prior art relied upon by the party claiming invalidity. Laser Alignment, Inc. v. Woodruff & Sons, Inc., 491 F.2d 866, 871 (7th Cir. 1974). Thus for the purposes of determining obviousness on this appeal, the Foster patent will be presumed valid.
35 U.S.C. § 103 provides:
A patent may not be obtained . if the differences between the subject matter sought to be patented and the prior art are such that the subject matter as a whole would have been obvious at the time the invention was made to a person having ordinary skill in the art to which said subject matter pertains. Patentability shall not be negatived by the manner in which the invention was made.
In Graham v. John Deere Co., 383 U.S. 1, 86 S.Ct. 684, 15 L.Ed.2d 545 (1966), the Supreme Court outlined the proper analysis to be followed when obviousness is at issue.
While the ultimate question of patent validity is one of law [citation omitted], the § 103 condition, which is but one of three conditions, each of which must be satisfied, lends itself to several basic factual inquiries. Under § 103, the scope and content of the prior art are to be determined; differences between the pri- or art and the claims at issue are to be ascertained; and the level of ordinary skill in the pertinent art resolved.
Id. at 17, 86 S.Ct. at 694. “[T]he Graham analysis [is] the exclusive means by which to measure nonobviousness under section 103.” Republic Industries, Inc. v. Schlage Lock Co., 592 F.2d 963 (7th Cir. 1979).3
In its findings the district court stated that Lindsay did not offer “evidence of any kind on the level of ordinary skill at any time in any art of farm earth-working equipment.” The district court then concluded that because Lindsay had failed to establish evidence on one of the factual inquiries as required by Graham, the defendant had not overcome the presumption of validity. The district court’s conclusion would be correct if Graham required that in every case the level of skill in the pertinent [344]*344art must be resolved. This reading of Graham is overbroad.
The district court incorrectly relied on Universal Athletic Sales Co. v. American Gym, 546 F.2d 530 (3d Cir. 1976), in reaching its conclusion. In Universal the Third Circuit held that a district court could not invalidate a patent when the record was deficient with respect to evidence on the level of skill. But this rule is not without exception.
The correct rule was formulated by this court in Research Corp. v. Nasco Industries, Inc., 501 F.2d 358 (7th Cir.), cert. denied, 419 U.S. 1096, 95 S.Ct. 689, 42 L.Ed.2d 688 (1974). In Research Corp. a patent was held obvious on a motion for summary judgment. No evidence had been heard on the level of skill in the pertinent art. The court stated:
Here there was no factual dispute concerning the state of the prior art, as recorded in the Taylor article. The differences between the method there disclosed and the claimed invention are plainly discernible from the documents, . A full trial, for the sole purpose of hearing expert testimony on the level of ordinary skill in the art, could not have aided the court in resolving the legal question of obviousness, and the court could properly rule on that issue “without the taking of testimony from experts in the pertinent area of knowledge.” (citation omitted) Id. at 361.
The court further held that this method of determining obviousness was appropriate
. where the structure and mode of operation of the accused device may be readily comprehended by the court and compared with the invention described and claimed in the patent without need of technical explanation by expert witnesses. (citation omitted) Id. at 362.
The gist of our holding in that case is that if the differences between the patent in suit and the prior art are such that the subject matter as a whole is obvious to a layman, a determination of the level of skill on the basis of expert testimony in the pertinent art would be useless. What is obvious to a layman would certainly be obvious to one skilled in the art to which the subject matter pertains.
Ill
Having delineated the proper standard for evaluating obviousness, we turn to the question of the instant patent’s validity.
The Sigmund patent, U.S. Patent No. 3,574,320, is pertinent prior art relating to the Foster patent. The district court said as much when it found that the Sigmund patent was the “one and the same” harrow contained in a brochure which was before the patent examiner. On the Sigmund harrow, the teeth are attached to the rail at a ninety degree angle and are spring-urged toward the ground, much like the Foster harrow. The specifications of the Sigmund patent provide that the rail to which the teeth are attached “is disposed at a 45 degree angle with respect to the direction of travel. . . [This] insures that the teeth will slice through the plow furrows at a 45 degree angle instead of paralleling a continuous unbroken furrow [resulting] in a more efficient and effective mulching of the soil.” Plaintiff’s Exhibit 7. The Foster harrow which Blacksmith manufactures is constructed with the teeth at a forty-five degree angle to the rail. As can be seen from Diagram 3 the teeth plow through the ground at a forty-five degree angle with respect to the direction of travel in both the Sigmund and Foster harrows. Although it cannot be seen from the diagram, the teeth on both harrows are also tilted up from the ground at approximately a forty-five degree angle (see Diagram 2). Thus the teeth in Sigmund and Foster are in the ground in precisely the same attitude.
Further, the Foster patent provides that the teeth be spaced six inches apart and that the lower end of each tooth is beneath the upper end of an adjacent tooth. When [345]*345the teeth are pulled through the soil at a forty-five degree angle, each tooth disturbs a swath of ground wider than the tooth itself. Overlapping the teeth, i. e., placing the bottom of one tooth below the top of another, ensures that all of the ground between any two teeth is thoroughly mulched. This characteristic is merely a function of the length and spacing of the teeth. It is substantially met by the Sigmund patent where it is stated:
It is also preferred that the teeth be spaced apart on 5V2 inch centers. . It is also preferred that the teeth extend approximately 12 inches below the lower flange of the [rail]. Plaintiff’s Exhibit 7.
These specifications in conjunction with the fact that the rail is carried at a forty-five degree angle to the direction of travel ensures that all of the ground is disturbed by a harrow constructed in accordance with the Sigmund design as it is by the Foster harrow. With respect to the direction of travel (see Diagram 3), the bottom of each Sigmund tooth is “below” the upper end of an adjacent tooth.
Thus the Foster patent was designed in a way which achieves the identical results of the Sigmund patent. The teeth of the harrow are the only working parts which affect the soil. In Sigmund and Foster the rails are in different positions with respect to the direction of travel, but the teeth are always in the same position relative to the ground. Attaching the teeth to the rail at a forty-five degree angle and then adjusting the position of the rail are differences from the Sigmund structure that can only be characterized as obvious. No expert testimony from one skilled in the art of harrowing and mulching tillable ground was needed in the trial to prove this fact.
The argument does not end here. It is Blacksmith’s position that when the Foster harrow is turned twenty degrees from the direction of travel, it can be used behind a disc harrow, as well as a field cultivator. Blacksmith argues that the Foster harrow still disturbs all or effectively all of the soil when turned twenty degrees whereas the Sigmund harrow cannot be used effectively at this angle. Although this feature may be true, it cannot be found in the Foster patent. “[T]he claims of a patent limit the invention” and “. . . claims are to be construed in light of the specifications.” United States v. Adams, 383 U.S. 39, 48-49, 86 S.Ct. 708, 713, 15 L.Ed.2d 572 (1966). See also Laser Alignment, Inc. v. Woodruff & Sons, Inc., 491 F.2d 866, 872 (7th Cir. 1974) (claims of a patent are the measure of its grant). The patentability of the Foster invention cannot be based on a feature which is not articulated in the patent’s claim.
Finally, Blacksmith argues that we should weigh secondary considerations such as commercial success, unexpected and beneficial results, and imitations. See Graham, supra, 383 U.S. at 17-18, 86 S.Ct. 684. Although these considerations may be “indicia of obviousness or nonobviousness,” id. at 18, 86 S.Ct. 684, they may be weighed “[o]nly in a close case, in which application of the primary criteria of nonobviousness under section 103 does not produce a firm conclusion . , . .” Republic Industries, supra. Here the application of section 103’s criteria creates no doubt about a determination that the Foster patent is invalid for obviousness.
The judgment of the district court is reversed.
Diagram 1
[346]*346Diagram 3