A pair of congruences $\theta$ and $\theta^*$ are called factor congruences if
$\theta \vee \theta^*$ = full congruence. $\nabla$
$\theta \wedge \theta^*$ = trivial congruence. $\triangle$
I need to prove that every chain has only two factor congruences which are $\triangle$ and $\nabla$. But I do not seem to understand which property of chains can be used here to start with the proof.