The whole Rosary is composed of twenty decades.  Each decade is recited in honor of a mystery in Our Lord's Life and that of his Blessed Mother.  It is customary to recite five decades at a time while meditating on one set of mysteries.  The decades may be separated, if the entire chaplet is completed on the same day.  Each Mystery may be meditated "bead by bead" for every Hail Mary of the decade.