The main ceremony in the Catholic Church is the Mass; like many types of ceremonies, the events in a Mass happen in a prescribed order. The events in a Mass are divided into two categories: the Ordinary and the Proper. The Ordinary of the Mass consists of all of the parts of the Mass with unchanging text (for example, there are certain prayers that are recited in exactly the same way in every single Mass); the Proper of the Mass consists of all of the parts of the Mass with texts that change according to the season (for example, a priest will give a sermon in each Mass, but the actual words are different every time). Many church musicians chose to focus on composing music for the Mass Ordinary, because that music could be used over and over again.
The music of the early Catholic Church is called chant, plainchant, or Gregorian chant. 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.
There are several characteristics of chant:
The music of the early Catholic Church is called a hymn
Catholic mass has no perscribed order.