If H is a subgroup of a group G, then right cosets are not always the same
as left cosets; that is, it is not always the case that gH = Hg for all g ∈ G.
The subgroups for which this property holds play a critical role in group
theory: they allow for the construction of a new class of groups, called factor
or quotient groups. Factor groups may be studied by using homomorphisms,
a generalization of isomorphisms.