  For a finite group $G$, we introduce a generalization of norm relations in
the group algebra $\mathbb Q[G]$. We give necessary and sufficient criteria for
the existence of such relations and apply them to obtain relations between the
arithmetic invariants of the subfields of a normal extension of algebraic
number fields with Galois group $G$. On the algorithmic side this leads to
subfield based algorithms for computing rings of integers, $S$-unit groups and
class groups. For the $S$-unit group computation this yields a polynomial time
reduction to the corresponding problem in subfields. We compute class groups of
large number fields under GRH, and new unconditional values of class numbers of
cyclotomic fields.
