Graver Tank & Mfg. Co. v. Linde Air Products Co., 339 U.S. 605 (1950)
歷史背景及判決影響：
均等論(doctrine of equivalents)之濫觴為 1853 年美國最高法院的Winans v. Denmead判決 1 ，雖然該案並未用到「均等論」這三個字，但法院已將專利範圍擴張至涵蓋系爭產品 2，並闡明其原因是因為系爭產品與專利之間無實質差異。3
「文義侵權」與「均等侵權」則是在Graver Tank案才被最高法院清楚地劃分開來了，並將三部測試法(function-way-result tripartite test)與均等論連結在一起，此後三部測試法成為判斷均等最重要的原則之一。事實上，三部測試法早在 1817 年的Gary v. James案就已經面世，但當時是用在一般侵權(general infringement)的判斷上 (當時法院對於請求項的判斷並不如現在嚴謹，雖然沒有均等侵權的概念，但也並非純粹的文義侵權)。
(均等論直到 1997 在Warner-Jenkinson一案中才又被最高法院討論，並說明三部測試法及非實質差異法各有優缺點，而已知的可替換性(known interchangeability)也相當重要，重點在於「被控侵權之產品或製程是否包括與請求項中每一個要件相同或均等的元件」)
判決重點：
- 當兩裝置以實質相同的方式執行相同的工作，並達到實質相同的結果，它們就是均等的。4
- 當判斷均等時，必須注意該成分被用在專利中的目的、與其他成分結合時的性質、要執行的功能，以及PHOSITA是否會知道其之間的可置換性(interchangeability)5
- 有兩位法官提出不同意見書，說明專利法要求專利權人明確定義其發明，此範圍對公眾來說應是不可調整的，況且說明書中已揭露錳矽酸鹽是可替換使用的的焊接成分，若原告並未請求此權利範圍，應視為貢獻給公眾(但在當時貢獻原則並非主流)。
本案歷程：
本案之系爭專利為US2,043,960，專利中揭露之電銲劑或助鎔劑由鹼土族矽酸鹽與氟化鈣所組成(原告之產品為鈣與鎂(鹼土族元素)之矽酸鹽)，而被告之產品為鈣與錳(非鹼土族元素)之矽酸鹽，顯見被告產品並未落入請求項的文義中。
最高法院引用Winans案及另一件Sanitary案，並說明均等論也可適用於化合物專利。由專家證人的證無可以知道，錳與鎂的化學反應十分相似，且沒有證據顯示此技術是被告獨立開發出來的。最高法院最終判決PHOSITA已經知道可以將鎂置換為錳，並依非實質差異法判決構成侵權。6
- Winans發明了一種可載運較多煤塊的車廂，此種車廂是圓椎狀結構，讓部分重量分散到側邊而非全部由底部承受，而被告的車廂是八角椎狀，但原理類似。最高法院認為不可能有人能製造出一個絕對的圓錐狀結構，判決被告的產品與圓錐狀近似故侵權。
- The patentee, not exaggerating the theoretical superiority of the form of his car, overlooked those facts which reduced its practical value to the level of cars of a form widely variant from his own. The object of this suit is to repair that defect of observation. It is that this Court shall extend, by construction, the scope and operation of his patent, to embrace every form which in practice will yield a result substantially equal or approximate to his own.
- that the octagon car, practically, was as good as the conical ones; and that, substantially, the witness saw no difference between the two.
- The doctrine of equivalents evolved in response to this experience. The essence of the doctrine is that one may not practice a fraud on a patent. Originating almost a century ago in the case of Winans v. Denmead, 15 How. 330, it has been consistently applied by this Court and the lower federal courts, and continues today ready and available for utilization when the proper circumstances for its application arise. “To temper unsparing logic and prevent an infringer from stealing the benefit of the invention,” [Footnote 1] a patentee may invoke this doctrine to proceed against the producer of a device “if it performs substantially the same function in substantially the same way to obtain the same result.” Sanitary Refrigerator Co. v. Winters, 280 U. S. 30, 280 U. S. 42.
- Consideration must be given to the purpose for which an ingredient is used in a patent, the qualities it has when combined with the other ingredients, and the function which it is intended to perform. An important factor is whether persons reasonably skilled in the art would have known of the interchangeability of an ingredient not contained in the patent with one that was.
- The question which thus emerges is whether the substitution of the manganese, which is not an alkaline earth metal, for the magnesium, which is, under the circumstances of this case, and in view of the technology and the prior art, is a change of such substance as to make the doctrine of equivalents inapplicable; or, conversely, whether, under the circumstances, the change was so insubstantial that the trial court’s invocation of the doctrine of equivalents was justified.