delivered the opinion of the Court.

Two courts below have concurred in holding three patent claims to be valid,^[1] and it is stipulated that, if valid, they have been infringed. The issue, for the resolution of which we granted certiorari,^[2] is whether they applied correct criteria of invention. We hold that they have not, and that by standards appropriate for a combination patent these claims are invalid.

[*149] Stated without artifice, the claims assert invention of a cashier’s counter equipped with a three-sided frame, or rack, with no top or bottom, which, when pushed or pulled, will move groceries deposited within it by a customer to the checking clerk and leave them there when it is pushed back to repeat the operation. It is kept on the counter by guides. That the resultant device works as claimed, speeds the customer on his way, reduces checking costs for the merchant, has been widely adopted and successfully used, appear beyond dispute.

The District Court explicitly found that each element in this device was known to prior art. “However,” it found, “the conception of a counter with an extension to receive a bottomless self-unloading tray with which to push the contents of the tray in front of the cashier was a decidedly novel feature and constitutes a new and useful combination.” ^[3]

The Court of Appeals regarded this finding of invention as one of fact, sustained by substantial evidence, and affirmed it as not clearly erroneous. It identified no other new or different element to constitute invention and overcame its doubts by consideration of the need for some such device and evidence of commercial success of this one.

Since the courts below perceived invention only in an extension of the counter, we must first determine whether they were right in so doing. We think not. In the first place, the extension is not mentioned in ¿he claims, except, perhaps, by a construction too strained to be consistent with the clarity required of claims which define the boundaries of a patent monopoly. 38 Stat. 958, 35 U. S. C. § 33; United Carbon Co. v. Binney & Smith Co., 317 U. S. 228; General Electric Co. v. Wabash Corp., 304[*150] U. S. 364. In the second place, were we to treat the extension as adequately disclosed, it would not amount to an invention. We need not go so far as to say that invention never can reside in mere change of dimensions of an old device, but certainly it cannot be found in mere elongation of a merchant’s counter — a contrivance which, time out of mind, has been of whatever length suited the merchant’s needs. In the third place, if the extension itself were conceded to be a patentable improvement of the counter, and the claims were construed to include it, the patent would nevertheless be invalid for overclaiming the invention by including old elements, unless, together with its other old elements, the extension made up a new combination patentable as such. Bassick Mfg. Co. v. Hollingshead Co., 298 U. S. 415, 425; Carbice Corp. v. American Patents Development Corp., 283 U. S. 27. Thus, disallowing the only thing designated by the two courts as an invention, the question is whether the combination can survive on any other basis.- What indicia of invention should the courts seek in a case where nothing tangible is new, and invention, if it exists at all, is only in bringing old elements together?

While this Court has sustained combination patents,^[4] it never has ventured to give a precise and comprehensive definition of the test to be applied in such cases. The voluminous literature which the subject has excited discloses no such test.^[5] It is agreed that the key to patent-[*151] ability of a mechanical device that brings old factors into cooperation is presence or lack of invention. In course of time the profession came to employ the term “combination” to imply its presence and the term “aggregation” to signify its absence, thus making antonyms in legal art of words which in ordinary speech are more nearly synonyms. However useful as words of art to denote in short form that an assembly of units has failed or has met the examination for invention, their employment as tests to determine invention results in nothing but confusion. The concept of invention is inherently elusive when applied to combination of old elements. This, together with the imprecision of our language, have counselled courts and text writers to be cautious in affirmative definitions or rules on the subject.^[6]

The negative rule accrued from many litigations was condensed about as precisely as the subject permits in Lincoln Engineering Co. v. Stewart-Warner Corp., 303 U. S. 545, 549: “The mere aggregation of a number of old parts or elements which, in the aggregation, perform or produce no new or different function or operation than that theretofore performed or produced by them, is not patentable invention.” To the same end is Toledo [*152] Pressed Steel Co. v. Standard Parts, Inc., 307 U. S. 350, and Cuno Engineering Corp. v. Automatic Devices Corp., 314 U. S. 84. The conjunction or concert of known elements must contribute something; only when the whole in some way exceeds the sum of its parts is the accumulation of old devices patentable. Elements may, of course, especially in chemistry or electronics, take on some new quality or function from being brought into concert, but this is not a usual result of uniting elements old in mechanics. This case is wanting in any unusual or surprising consequences from the unification of the elements here concerned, and there is nothing to indicate that the lower courts scrutinized the claims in the light of this rather severe test.

Neither court below has made any finding that old elements which made up this device perform any additional or different function in the combination than they perform out of it. This counter does what a store counter always has done — it supports merchandise at a convenient height while the customer makes his purchases and the merchant his sales. The three-sided rack will draw or push goods put within it from one place to another — just what any such a rack would do on any smooth surface— and the guide rails keep it from falling or sliding off from the counter, as guide rails have ever done. Two and two have been added together, and still they make only four.

Courts should scrutinize combination patent claims with a care proportioned to the difficulty and improbability of finding invention in an assembly of old elements. The function of a patent is to add to the sum of useful knowledge. Patents cannot be sustained when, on the contrary, their effect is to subtract from former resources freely available to skilled artisans. A patent for a combination which only unites old elements with no change in their respective functions, such as is presented here, obviously withdraws what already is known into[*153] the field of its monopoly and diminishes the resources available to skillful men. This patentee has added nothing to the total stock of knowledge, but has merely brought together segments of prior art and claims them in congregation as a monopoly.

The Court of Appeals and the respondent both lean heavily on evidence that this device filled a long-felt want and has enjoyed commercial success. But commercial success without invention will not make patentability. Toledo Pressed Steel Co. v. Standard Parts, Inc., supra. The courts below concurred in finding that every element here claimed (except extension of the counter) was known to prior art. When, for the first time, those elements were put to work for the supermarket type of stores, although each performed the same mechanical function for them that it had been known to perform, they produced results more striking, perhaps, than in any previous utilization. To bring these devices together and apply them to save the time of customer and checker was a good idea, but scores of progressive ideas in business are not patentable, and we conclude on the findings below that this one was not.

It is urged, however, that concurrence of two courts below, in holding the patent claims valid, concludes this Court. A recent restatement of the “two-court rule” reads, “A court of law, such as this Court is, rather than a court for correction of errors in fact finding, cannot undertake to review concurrent findings of fact by two courts below in the absence of a very obvious and exceptional showing of error.” Graver Tank Co. v. Linde Co., 336 U. S. 271, 275. The questions of general importance considered here are not contingent upon resolving conflicting testimony, for the facts are little in dispute. We set aside no finding of fact as to invention, for none has been made except as to the extension of the counter, which[*154] cannot stand as a matter of law. The defect that we find in this judgment is that a standard of invention appears to have been used that is less exacting than that required where a combination is made up entirely of old components. It is on this ground that the judgment below is

Reversed.