Take 
a number field and 
a divisor of 
. A congruence subgroup 
is defined as a subgroup of the group of all fractional ideals
relative prime to 
(
)
that contains all principal ideals that are generated by elements of 
that are equal to 1 (mod 
). These principal ideals split completely in all Abelian extensions
and are consequently part of the kernel of the Artin map
for each Abelian extension 
.
When there exists an Abelian extension 
such that 
contains all the primes that ramify in 
and such that 
equals the kernel of the Artin map,
then 
is called the class field of 
.
To formulate the main theorems, the equivalence relation on congruence subgroups is needed, namely that 
and 
are called equivalent if there exists a divisor 
such that 
.
Class field theory consists of two basic theorems. The existence theorem states that to every equivalence class of congruence subgroups, there belongs a class field 
. The classification theorem states that
for each number field 
, there is a unique one-to-one
correspondence between the Abelian extensions 
and the equivalence classes of congruence subgroups 
.
This is important because this means that all Abelian extensions of a number field can be found using a property that is completely determined within the number field itself.
