A reissue application is a means for an applicant to correct an error in a patent. The error is usually a defect in the specification or the drawings, but the error could also be that applicant's claims were too broad or too narrow in comparison to what the applicant could rightfully claim.
Ironically, applicant's choice to cancel a broad claim in favor of a narrow claim in order to avoid the prior art is not an error that can be corrected by a narrowing reissue; however, a patent attorney's "failure to appreciate the full scope of the invention" generally is recognized as an error justifying a reissue with broadened claims. Note that any claims sought through a reissue that would broaden the scope of the original claims must be applied for within two (2) years of the patent issuing.
If a reissued patent is allowed, the original patent is "surrendered" in favor of the revised version. The revised version is enforceable from its own date of issue, but it expires when the original patent would have expired. However, if any claims from the original patent are carried forward in substantially identical form in the reissued patent, such claims are enforceable as though they had been in effect from the issue date of the original patent.
Ironically, applicant's choice to cancel a broad claim in favor of a narrow claim in order to avoid the prior art is not an error that can be corrected by a narrowing reissue; however, a patent attorney's "failure to appreciate the full scope of the invention" generally is recognized as an error justifying a reissue with broadened claims. Note that any claims sought through a reissue that would broaden the scope of the original claims must be applied for within two (2) years of the patent issuing.
If a reissued patent is allowed, the original patent is "surrendered" in favor of the revised version. The revised version is enforceable from its own date of issue, but it expires when the original patent would have expired. However, if any claims from the original patent are carried forward in substantially identical form in the reissued patent, such claims are enforceable as though they had been in effect from the issue date of the original patent.