Prior use of an invention is only a bar to a patent if it occurs inside the United States and this occurred either before the applicant’s invention or more than one year before the applicant’s date of filing its United States patent application.

There is no anticipation if a prior product merely could have been used for the claimed process [87] or only a minor modification would be needed to bring it within the scope of the claims. [88] On the other hand, merely discovering new properties of something that was used before or failing to appreciate the results of what one was doing does not avoid anticipation. [89]

A difficulty arises in cases where it transpires that someone had previously done what was claimed, but had done so by accident. An example of this would be a process carried out in a particular way so that a small amount of a particular product is produced by chance rather than by design. In this situation, the case law holds that there is in general no anticipation. [90] The doctrine is, however, strictly applied to situations where the prior use was truly fortuitous and it is always difficult to predict whether a particular situation will be regarded as accidental or whether the courts will take the view that what one now seeks to claim was inherent in the prior use. [91]

The simple issue of secret prior use or experimental use by another rarely comes before the courts, since a situation in which these issues arise will almost always also raise the question of who first invented the subject matter in question and a decision on this point normally disposes of the case.

There is one situation in which the question of prior use is of importance in its own right. This is when the prior inventor in fact did not develop his invention. As will be discussed later, under the "prior invention" provisions of the statute the earlier inventor will prevail only if he has not abandoned his invention. No such limitation exists in respect of making a finding of anticipation based on prior use. The law is still not fully developed, but it appears that even though a "mere abandoned experiment" does not constitute anticipatory prior use, [92] prior use can be anticipatory if it is fully developed in the United States. This is so even if no steps are taken actively to bring it to the notice of the public and even though the device is soon superseded by an improvement. [93]

In order to be anticipatory under 35 USC § 102(b), a prior use must be "public". As under 35 USC § 102(a), it must also have been a use of the complete invention. It has been held that the public use requirement of 35 USC § 102(b) is met if the invention is open to the public so that any member of the public can see it if he so desires. However, "the party asserting patent invalidity on the basis of [prior] public use ... must show such use by evidence that is clear and convincing." [94] Use by one member of the public without that use informing other members of the public of the true nature of the invention has been held to be sufficient to constitute a bar under this section. [95] The Federal Circuit has held that "any use of [the] invention by a person other than the inventor who is under no limitation, restriction or obligation of secrecy to the inventor" is a public use. [96] However, case law on what is meant by "public" is sometimes difficult to follow. For example permitting use by co-workers of a toy unrelated to the employer's business has been held by the Federal Circuit not to be a public use [97], whereas disclosure to a party of one's friends was held by the same court to be a public use. [98] The court based its differing conclusions on the different degrees of "control" that the inventor exercised over the use by others in the two cases.

Furthermore, any commercial use of a process by the patentee or patent applicant within the United States more than one year prior to the date of filing a patent application is deemed to be a public use. [99] However, secret commercial use by a third party who developed the invention independently may not act as a bar to the grant of a patent. [100] It may, however, be difficult to persuade a court that a commercial use was not public unless positive steps to maintain secrecy were adopted even if the process in question is carried out entirely within a third party's plant. [101] There is a policy reason why the courts have come to different conclusions as to why commercial secret use (for example where the invention relates to the process for manufacture of a product and the nature of the invention cannot be determined from the product as sold) is a bar if done by the inventor but not if done by a third party. The policy view taken by the courts has been that the purpose of the patent system is to encourage early filing of patent applications. To allow an inventor to use his or her invention commercially and only file a patent application when a competitor comes on to the market would have the effect of extending the effective term of the patent since in addition to the statutory term, the patentee would also benefit form his or her pre-patent filing commercialization. No such factors arise in respect of commercial use by a third party. Of course, if the products of the commercialization reveal the nature of the invention other grounds for denying patentability come into play.

In cases where the prior use was of an essentially experimental nature [102] it has been held that experimental use does not fall within the definition of public use. The fact that a use was experimental may excuse an act that would otherwise constitute a prior use, for example giving a sample to a member of the public without any express imposition of confidence, at least in cases where it was necessary to carry out experimentation in a way that involved members of the public. [103] Similarly, there was no bar when a patented outdoor lighting system had been used in a public rest area on a highway in a case where public testing was needed to determine the durability of the system in real life conditions and the patentee had done nothing to lead the public to believe the invention was being put into the public domain. [104] On the other hand, to succeed with a claim that a use was experimental probably requires that he who claims to be the experimenter retains some degree of control over the "experimentation" and keeps some records as to what develops. [105]

The purpose of the "experimental use exception" is to give the inventor the opportunity to reduce the invention to practice so that once this has occurred, subsequent uses no longer qualify for the exception. [106] This fact severely limits the possibility of claiming that a prior use was experimental in connection with design patents since a design is reduced to practice as soon as an embodiment is constructed. [107] However, if the experimentation was confined to functional aspects of the device then an exception may still exist. [108]

Since public prior use may be difficult to prove, the courts have held that "corroboration is required of any witness whose testimony alone is asserted to invalidate a patent regardless of his or her level of interest." [109]