Obviousness-type non-statutory double patenting is a judicially created doctrine that operates to prevent patentees from improperly extending a patent's effective term through successive patent grants.
The non-statutory double patenting doctrine was created at a time when patent term was measured from the grant date. At that time, it was easy to "play games" with the patent terms by using a series of continuation applications. Because patent term (for new patents) is now defined by the filing/priority date, most of the potential for games has been eliminated because a family of related applications will typically have the same term. However, a growing issue is patent term adjustment (PTA) that increases the patent term for applications with long prosecution pendency. Because of the increased backlog and changes to PTA calculations, it is likely that later-issued applications in a family will have a longer PTA.
Courts and the PTO have agreed that a patentee can overcome a non-statutory double patenting problem by filing a terminal disclaimer that ties the patent-term of a later-granted patent to the patent term of an earlier-granted patent.
The impact of a terminal disclaimer depends upon what was actually disclaimed. Although applicants can draft their own terminal disclaimer, most folks use the form provided by the USPTO. (SB-0025). The PTO Form disclaims the term of the later-issued patent that extends beyond the earlier-issued patent's term. Regarding PTA, the Form expressly ties the later-issued patent's term to the PTA-adjusted term of the earlier-issued patent.
Although not expressly stated, the Form seems to imply that the PTA of the later-issued patent can be cut-short by the disclaimer. That result, however, is not compelled by the law. In theory (and largely in practice) PTA is based on patent office delay. As such, the equitable basis of the non-statutory double patenting doctrine might not apply to require disclaimer of PTA.
A patent applicant may want to pursue a test-case by modifying the terminal disclaimer form to expressly state that the disclaimer does not cut short the PTA of that later-issued patent. Thus, the term of a later-issued patent with a 400-day PTA and a terminal-disclaimer would limited to the term of the earlier-issued patent plus 400-days.