Classically, there are fifteen decades:
5 Joyful: The Annuntiation; the Visitation; the Nativity; the Presentation, the Finding in the Temple
5 Sorrowful: The Agony in the Garden, the Scourging at the Pillar, the Crowning with Thorns, The Carrying of the Cross, and the Death of Jesus on the Cross;
5 Glorious: The Resurrection, the Ascention, the Descent of the Holy Spirit, the Assumption (of the BVM), and the Coronation of the BVM.
Pope John Paul II added 5 Luminous:
The Baptism in the Jordan, the Miracle at Cana, the Proclamation of the Kingdom, the Transfiguration, and the Insitution of the Blessed Eucharist