The book used in this exhibit is a 15th-century gradual from a monastery in Spain and is one volume of a set of seven. A gradual contains the chants for the Eucharist or Mass and would have been in daily use. You can get a sense of how frequently it was used, by looking at the lower right-hand corner of this page—the discoloration is from the transfer of dirt and oils from the monks who used this book.
But why is it so big? Rather than create separate books of music for each member of the choir, a costly and time-consuming process, one large book was created, allowing the entire choir to share. Likely newer members would have been positioned closest to the book since they would need more guidance from the book than more experienced members.
As big as this book is, it still wasn’t big enough to hold an entire year’s worth of chants and is part of a set of seven volumes. Unfortunately, two are lost to time, but Loras holds the remaining five.