  We show that we can develop from scratch and using only classical language a
theory of relative quadratic extensions of a given number field $K$ which is as
explicit and easy as for the well-known case that $K$ is the field of rational
numbers. As an application we prove a reciprocity law which expresses the
number of solutions of a given quadratic equation modulo an integral ideal
$\mathfrak{a}$ of $K$ in terms of $\mathfrak{a}$ modulo the discriminant of the
equation. We study various $L$-functions associated to relative quadratic
extensions. In particular, we define, for totally negative algebraic integers
$\Delta$ of a totally real number field $K$ which are squares modulo~$4$,
numbers $H(\Delta,K)$, which share important properties of classical Hurwitz
class numbers. In an appendix we give a quick elementary proof of certain
deeper properties of the Hilbert symbol on higher unit groups of dyadic local
number fields.
