= N-variable Lindenbaum algebra
{title2=$F_n/E_n(T)$}
Take formulas whose <free variables> lie among a fixed $n$-tuple and identify them when their universal equivalence follows from $T$. <Conjunction>, disjunction and <negation> induce a <Boolean algebra>; its order is implication modulo $T$. <Complete types> correspond to <ultrafilters> and isolated <complete types> to <atom of a Boolean algebra>[atoms]. For a <complete theory> in a <countable> language, finiteness of these algebras for every finite $n$ characterizes <countable> categoricity.
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