Abstract: Nearly every aspect of words is subject to alteration: pronunciation, spelling, part of speech, and definition. These aspects are often altered intentionally, yet speakers can err in each of them when using words. Using the semantics of ‘equality’ and ‘equity’ as an example, this paper resolves the puzzle by suggesting that such alterations are negotiated between speaker and audience, and introduces a formal framework from financial theory governing those negotiations. The model suggests that successful lexical negotiations are constrained by transaction costs, which implies that some conceptual engineering projects (like Haslanger’s revision of the word ‘woman’) are likely to fail even though the semantic revision of ‘equality’ and ‘equity’ was largely successful.