A Choirbook is a large format manuscript used by choirs in churches or cathedrals during the Middle Ages and Renaissance. The book is large enough for the entire choir to read from one book. Often for polyphonic works all the musical parts or voices of a piece of music are presented on a single pair of pages, in contrast to partbook, where each part is in a separate book. Choirbooks were generally put on a stand with the smaller boy sopranos in front and the men in back. Once the printing of music became easier and more commonplace, choirbooks fell out of favor, replaced by smaller, cheaper, and easier to handle books and octavos.
A choirbook was a major investment. Many of them were stark and utilitarian and show signs of heavy and constant use. At larger cathedrals choirbooks were sometimes lavishly decorated and illuminated. They represent a major investment, and were rarely owned by single people, but rather families or institutions.
Read more about Choirbook: Major Choirbooks