The entire church service was sung, so musicians and church officials were responsible for knowing a large number of chants and for choosing which ones to use in each church service. Pope Gregory the Great decided that since some chants were appropriate for Christmas, and some were appropriate for Easter, and others were appropriate for various feast days, the chants should be tied to the church calendar. This meant that all 3,000 or so chants had to be written down and organized. Because Pope Gregory is credited with starting this project, chant is often referred to as Gregorian chant. The written notation of many of these chants survived so that we can study them today.