Patent claim construction involves an analysis of intrinsic evidence primarily, and then extrinsic evidence as needed.
Intrinsic evidence includes the following, in this priority order: patented claims, patent specification, and prosecution history. With regard to the prosecution history, this includes not only the prosecution history of the reviewed patent, but also the prosecution histories of all US family members — that is, prior related US applications, prior related CIPs, related US siblings, related US children, and related US grandchildren. For prosecution history review, focus is placed on office actions and office action responses, and reviews look for the possible presence of two types of estoppel: 1) Argument-based prosecution history estoppel where the applicant explicitly indicates claim interpretation through arguments made in office action responses; and 2) Amendment-based prosecution history estoppel where a narrowing amendment is “made for a ‘substantial reason related to patentability’ when the record does not reveal the reason for the amendment.” (Festo)
Extrinsic evidence includes, for example: dictionaries and expert / inventor testimony. Extrinsic evidence may be considered to inform claim construction based upon the intrinsic evidence, but in most situations intrinsic evidence alone suffices to construe claims, and in many cases it is improper to rely on extrinsic evidence at all.