  No formal machinery for dealing with sense, as opposed to reference, is proposed in Frege 1892. But Frege defined the terms under which further discussion took place. There are two distinct but related notions, sense and reference. Equality plays a fundamental role, and a central issue is the substitutivity of equals for equals. Names, signs, expressions, can be equal in designation, but not equal in sense. There are both direct or extensional, and indirect or intensional contexts, and reference matters for the first while sense is fundamental for the second.