The written description requirement under 35 U.S.C. 112(a) mandates that the specification must describe the claimed invention in sufficient detail to demonstrate to a person skilled in the art that the inventor had possession of the claimed invention at the time of filing. According to MPEP 2166, a rejection based on the written description requirement can be made using the following form paragraph: "Claim [1] rejected under 35 U.S.C. 112(a) or pre-AIA 35 U.S.C. 112, first paragraph, as failing to comply with the written description requirement. The claim(s) contains subject matter which was not described in the specification in such a way as to reasonably convey to one skilled in the relevant art that the inventor or a joint inventor, or for applications subject to pre-AIA 35 U.S.C. 112, the inventor(s), at the time the application was filed, had possession of the claimed invention." This rejection is used when the examiner believes that the specification does not provide adequate support for the claimed subject matter, including situations where new matter has been introduced into the claims. To learn more: 35 U.S.C. 112(a) Written Description Patent Specification New Matter