Okay, this explains it...I think:
The rosary provides a physical method of keeping count of the number of Hail Marys said. The fingers are moved along the beads as the prayers are recited. By not having to keep track of the count mentally, the mind is more able to meditate on the mysteries. A five decade rosary contains five groups of ten beads (a decade), with additional large beads before each decade.[24] The Hail Mary is said on the ten beads within a decade, while the Our Father is said on the large bead before each decade. A new mystery is meditated upon at each of the large beads. Some rosaries, particularly those used by religious orders, contain 15 decades, corresponding to the traditional 15 mysteries of the rosary. Both five and 15 decade rosaries are attached to a shorter strand, which starts with a crucifix followed by one large, three small, and one large beads before connecting to the rest of the rosary.