For a commutative diagram of two short exact sequences joined by vertical maps f,g,h, there is a natural connecting map δ: ker h → coker f making the 6-term sequence ker f → ker g → ker h → coker f → coker g → coker h exact.
For a commutative diagram of two short exact sequences joined by vertical maps f,g,h, there is a natural connecting map δ: ker h → coker f making the 6-term sequence ker f → ker g → ker h → coker f → coker g → coker h exact.