Warner-Jenkinson Company v. Hilton Davis Chemical Co. 520 U.S. 17 (1997) Hilton invented a process to purify dyes, and got a patent on it. The patent specified that a solution used in the process must have a pH between 6.0 and 9.0. The 9.0 limit was added so that Hilton's process wouldn't conflict with a different patent that claimed 9.0+. But Hilton never said why they chose 6.0 as their lower limit. Warner came along and developed a process that used a solution at pH 5.0. Hilton sued for infringement. Warner argued that 5.0 was outside of the range that Hilton claimed in their patent. Warner argued that Hilton couldn't figure out how to make the process work below 6.0. Hilton countered that they made the process work all the way down to 2.2, but chose 6.0 as their lower limit for no particular reason. Hilton argued that Warner's process was still infringing because of the Doctrine of Equivalents (aka non-textual infringement). The Doctrine of Equivalents says that a product or process that does not literally infringe upon the express terms of a patent claim may nonetheless be found to infringe if there is an "equivalence" between the elements of the accused product or process and the claimed elements of the patented invention. See Graver Tank v. Linde Air Products Co. (339 U.S. 605 (1950)). The Trial Court found for Hilton. Warner appealed. Warner argued that the Doctrine of Equivalents (an old common law doctrine) was no longer good law because the Patent Act of 1952 (specifically 35 U.S.C. §112 ¶6) made it obsolete. Basically, the purpose of the Doctrine of Equivalents was correct errors in the patent process for people who didn't claim everything they should have claimed. But under the new law there was a provision to reissue patents, and so it was no longer needed. The Appellate Court affirmed. Warner appealed. The Appellate Court found that the Doctrine of Equivalents still existed and the only issue was whether substantial differences existed between Warner's process and Hilton's. That was a question of fact for a jury to decide. The US Supreme Court vacated and remanded. The US Supreme Court found that Congress had not explicitly intended to get rid of the Doctrine of Equivalents in §112 ¶6. The Court found that when applying the Doctrine of Equivalents, you have to look at every limitation in the patent and compare it to the infringer. Every single limitation must be either literally infringed or infringed by equivalence. That's known as the All Elements Rule (aka the All Limitations Rule). The Court found that the Doctrine of Prosecution History Estoppel holds that when a claim limitation is added in order get around prior art, then a patentee could not use the Doctrine of Equivalents to extend the bounds of the claim they previously limited. The Court found that if Warner can prove the reason they chose 6.0 as their lower bound was not to limit the patent, then infringement was still possible. The Court remanded the case to determine if Warner could explain their choice of pH limit. Basically, if it turned out that Warner chose 6.0 as their limit because they never figured out how to make the process work at a lower pH, or some other good reason like that, then they don't get the benefit of the Doctrine of Equivalents. On the other hand, if they just pulled that number out of thin air and could show that Hilton's process was substantially the same as their process, then the Doctrine of Equivalents would apply. Warner limited is scope to <9.0 to get around the prior art, but that didn't require him to limit his scope to >6.0 to get around the prior art. Warner argued that patents can be reissued to fix claims, and patents must meet a definiteness requirement. Therefore there was no reason to keep the Doctrine of Equivalents because Hilton should have been more definite about the outer limits of their invention, and if they didn't like their original limits, they could always go back and try to reissue the patents. However, the Court felt that the existence of these other methods to fix a patent claim didn't mean that the Doctrine of Equivalents wasn't still another option.