In this work we develop, through a governing field, genus theory for a number field $\K$ with tame ramification in $T$ and splitting in $S$, where $T$ and $S$ are finite disjoint sets of primes of $\K$. This approach extends that initiated by the second author in the case of the class group. It allows expressing the $S$-$T$ genus number of a cyclic extension $\L/\K$ of degree $p$ in terms of the rank of a matrix constructed from the Frobenius elements of the primes ramified in $\L/\K$, in the Galois group of the underlying governing extension. For quadratic extensions $\L/\Q$, the matrices in question are constructed from the Legendre symbols between the primes ramified in $\L/\Q$ and the primes in $S$.