A diagram lemma which states that the above commutative diagram of Abelian groups and group homomorphisms with exact rows gives rise to an exact sequence
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This commutative diagram shows how the first commutative diagram (shown here in blue) can be modified to exhibit the long exact sequence (shown here in red) explicitly.
The map 
is called a connecting homomorphism and
describes a curve from the end of the upper row (
) to the beginning of the lower row (
), which suggested
the name given to this lemma.
The snake lemma is explained in the first scene of Claudia Weill's film It is My Turn (1980), starring Jill Clayburgh and Michael Douglas.
