What is "antecedent basis"?

Claims in a patent are written in a specific form of legalese. The first time an element of a claim is mentioned, it must be introduced by the article "a" or "an" in front of it. This is known as "antecedent basis." If the same element is referenced again in later in the claim, then the article "the" must be used. If the patent applicant references an element in a claim that has not yet been introduced (using "a" or "an"), the Patent Examiner will reject the claim under 35 U.S.C. Section 112 for lacking sufficient antecedent basis.