Definition 17.2. Let $\phi (x,y)$ be an $L$-formula, $x,y$ types of finite length.

We say $\phi (x,y)$ has the order property with respect to $T$ if there is some $M\models T$, ${(a_i)}_{i\ge 0}$, ${(b_j)}_{j\ge 0}$ such that $M\models \phi (a_i,b_j)$ if and only if $i<j$.