At the Certosa di Pavia, I bought a copy (about 18x24 inches) of this page of ancient music, which they were selling at the gift shop of the monastery.  In the old churches they have the choir seats and because they couldn't print music before the 1400s, they wrote it on large pieces of sheep skins, and then they greatly embellished each page, which were bound in a large book. The pages of the book were about 18x24 inches, the same size as the copy I bought.  They put the book on a large stand and all the monks could sing from it.  This is a page about the birth of Jesus and the music notation is shown on the 4 lines, the square notes show how to go up and down, and the words are in Latin. This is how Gregorian of chant was written and it is still being sung. In the old abbey Mirasole, the French monks still chant every day, several times.  As I have written, two sit on one side of the church and one on the other, in the choir seats.  Two sing and the one on the other side answers the chant, and they are usually the only people in the cold dark church, illuminated by candles.