  This paper is concerned with the model-theoretic study of pairs $(K,F)$ where
$K$ is an algebraically closed field and $F$ is a distinguished subfield of $K$
allowing extra structure. We study the basic model-theoretic properties of
those pairs, such as quantifier elimination, model-completeness and saturated
models. We also prove some preservation results of classification-theoretic
notions such as stability, simplicity, NSOP$_1$, and NIP. As an application, we
conclude that a PAC field is NSOP$_1$ iff its absolute Galois group is (as a
profinite group).
