The oldest name for the devotion we know as the Rosary is "Our Lady's Psalter," a reference to its development from the custom of recitation of the psalms. It is customary for monks to recite the 150 psalms weekly. The early desert fathers are said to have recited the entire Psalter daily. In various times and places, private recitation of the psalms, or a third part of the psalms – that is, 50 – was enjoined on Christians. When a monk died, for instance, the priests of the community would say a Requiem Mass for their departed brother, but the monks who were not priests would recite 50 psalms in lieu of the Requiem Mass. Devout, educated laymen would also recite 50 psalms as a daily devotion. But this form of prayer was possible only for those who could read Latin and who could afford a personal copy of the Psalter. A form of daily prayer was also required for those who were poor or illiterate. Those who could not read the Psalms would instead recite 50 Paternosters, or repetitions of the Lord's Prayer.
Throughout history and across all religions, wherever prayers are repeated many times, counting devices have come into use to assist those who prayed. A sculpture from ancient Nineveh appears to show two winged women in an attitude of prayer, holding rosaries. Muslims use bead-strings to count the 99 names of God. One early Christian monk would gather 300 stones every morning and discard one with each prayer until he had discarded them all and fulfilled his daily obligation. Eastern monks use knotted cords to count 100 repetitions of the Jesus Prayer, "Lord Jesus Christ, Son of God, have mercy upon me, a sinner." The famous Lady Godiva, upon her death in the 11th century, bequeathed to a monastery "the circlet of precious stones which she had threaded on a cord in order that by fingering them one after another she might count her prayers exactly."
So it is not unexpected that strings of 50 beads came into popular use to count Paternosters. The devotion was sufficiently popular to support craft guilds of paternosterers all over 13th century Europe to manufacture these strings of prayerbeads.
Around the 12th century, the angel's salutation to Mary came into popular use as a devotion to be repeated 50 or 150 times: "Hail Mary, full of grace, the Lord is with thee. Blessed art thou among women and blessed is the fruit of thy womb." (The concluding petition, "Pray for us sinners now and at the hour of our death," was not appended to the Hail Mary until sometime later.) The Hail Marys were sometimes said in groups of 10, with bows and prostrations.
All of these streams came together in the preaching of the Dominican friar, Alan de Rupe (aka Alain de La Roche), in the 1470s. It was he who first promoted devotion to Our Lady's Psalter – the recitation of 150 Hail Marys. (It was also apparently he who first attributed the origin of the Rosary to St. Dominic, two centuries earlier.) Subsequently, Rosary confraternities associated with the Dominican Order began to spring up all over Europe to encourage and support the praying of Our Lady's Psalter. It was under their influence that the Rosary came to be standardized in its now-familiar form of 15 decades, each associated with a mystery from the life of Christ and his mother.
As regards the origin of the name, the Latin word rosarius means a garland or bouquet of roses. An early legend that traveled all over Europe connected this name with a story of Our Lady, who was seen to take rosebuds from the lips of a young monk when he was reciting Hail Marys and to weave them into a garland which she placed upon her head. Other names for the Rosary, corona and chaplet, also refer to Mary's crown of roses from this story.
The old English name for the Rosary, found in Chaucer and elsewhere was a "pair of beads." The word bead originally meant a prayer, and it is cognate with our word bid, meaning to beg, entreat, or pray. The more familiar meaning of bead referring to ornamental stones that are threaded together comes from the use of such beads to count prayers.
Fr. Kevin J. Scallon, an Irish Vincentian priest, wrote the following:
The greatness of the Rosary lies in its power to help us walk in the footsteps of Jesus. It draws us into those eternal moments in the life of Christ. In each mystery we gaze, as through a window, to contemplate with Mary the life and mysteries of her son. As at all her great shrines, Mary is never concerned to draw her children to herself but to her son. We recite the Hail Marys and gaze on Jesus who allows us to be with him at each moment from the Annunciation to the Crowning of his mother as Queen. . . . As we finger the beads and recite the Hail Marys, the words of Scripture pass before our mind and we drink from "the spring of living water welling up to eternal life" and "our hearts burn within us."