quality:correction
Short

The most common type of exact sequence is the short exact sequence. This is an exact sequence of the form

where ƒ is a monomorphism and g is an epimorphism. In this case, A is a subobject of B, and the corresponding quotient is isomorphic to C:

(where f(A) = im(f)).

A short exact sequence of abelian groups may also be written as an exact sequence with five terms:

where 0 represents the zero object, such as the trivial group or a zero-dimensional vector space. The placement of the 0's forces ƒ to be a monomorphism and g to be an epimorphism (see below).