Gregorian chant used the eight modes which came from the Byzantine chants. They were written down in a special musical notation called "neumes". These neumes do not show the exact rhythms, so that we cannot always be sure of the way they were sung. The rhythm was probably quite free and flexible. There were certainly no bar lines and regular beat. The chant was normally sung by men. Women only sang in convents, and even then they could not sing in all the services.