This book is about the ancient Music who it was written and stored. It was written in a scrawl impossible to decipher up to the thirteenth century, when Plain Song 1 (Plain Chant) made its appearance in square and diamond-shaped notes. The graduals and introits had not yet been reduced to bars, but the songs of the troubadours appear to have been in bars of three beats with the accent on the feeble note of each bar. However, the theory that this bar of three beats or triple time was used exclusively is probably erroneous. St. Isidore, in his treatise on music, speaking of how Plain Song should be interpreted, considers in turn all the voices and recommends those which are high, sweet and clear, for the execution of vocal sounds, introits, graduals, offertories, etc. This is exactly contrary to what we now do, since in place of utilizing these light tenor voices for Plain Song, we have recourse to voices both heavy and low.