 So one has to work a little to apply the 1933 definition to model-theoretic languages. There are basically two approaches: (1) Take one structure \(A\) at a time, and regard the nonlogical constants as constants, interpreted in \(A\). (2) Regard the nonlogical constants as variables, and use the 1933 definition to describe when a sentence is satisfied by an assignment of the ingredients of a structure \(A\) to these variables. There are problems with both these approaches, as Tarski himself describes in several places. The chief problem with (1) is that in model theory we very frequently want to use the same language in connection with two or more different structures – for example when we are defining elementary embeddings between structures (see the entry on  first-order model theory).  The problem with (2) is more abstract: it is disruptive and bad practice to talk of formulas with free variables being ‘true’. (We saw in Section 2.2 how Tarski avoided talking about truth in connection with sentences that have varying interpretations.) What Tarski did in practice, from the appearance of his textbook in 1936 to the late 1940s, was to use a version of (2) and simply avoid talking about model-theoretic sentences being true in structures; instead he gave an indirect definition of what it is for a structure to be a ‘model of’ a sentence, and apologised that strictly this was an abuse of language. (Chapter VI of Tarski 1994 still contains relics of this old approach.)