 A sentence \(S\) defines its class \(\Mod(S)\) of models. Given two languages \(L\) and \(L'\), we can compare them by asking whether every class \(\Mod(S)\), with \(S\) a sentence of \(L\), is also a class of the form \(\Mod(S')\) where \(S'\) is a sentence of \(L'\). If the answer is Yes, we say that \(L\) is reducible to \(L'\), or that \(L'\) is at least as expressive as \(L\). 