Said in Confession before absolution, and every night before sleep.
A prayer of sorrow for sin, and a firm purpose not to sin again.
When: In Confession, when the priest asks for it, and at night after the examen.
Why: Perfect contrition, sorrow out of love for God, forgives even grave sin when Confession is not yet possible, as long as we intend to go. It is worth knowing by heart for that reason alone.