 if every \(L_1\)-sentence \(\f\) is logically equivalent to some \(L_2\)-sentence \(\p\), i.e., \(\f\) and \(\p\) are true in the same models. Also, \(L_1\) and \(L_2\) have the same expressive power, \(L_1 \equiv L_2\), if \(L_1 \leq L_2\) and \(L_2 \leq L_1\), and \(L_2\) is stronger than \(L_1\), \(L_1 &lt; L_2\), if \(L_1 \leq L_2\) but \(L_2 \not\leq L_1\). Thus, \(L_1 &lt; L_2\) if everything that can be said in \(L_1\) can also be said in \(L_2\), but there is some \(L_2\)-sentence which is not equivalent to any sentence in \(L_1\).