A letter from patent counsel to a client providing an opinion that a patent is valid over the prior art patent and non-patent publications known to counsel. It discusses the materials counsel considered, including the subject patent, its file history and family, the known prior art, including any prior art search counsel conducted or commissioned, the level of ordinary skill in the art, claim construction, and the patent's novelty and nonobviousness under 35 U.S.C. §§ 102 and 103. It does not address other grounds for validity or any enforceability issues. The letter may serve as a starting point and may be tailored to suit the particular facts and situation. This Standard Document has integrated drafting notes with important explanations and drafting tips.