The choir books of the Bavarican State Library form a collection of 165 manuscripts with polyphonic music, mainly from the 16th and 17th centuries. Under the reign of Albrecht V (reigned 1550-1579) Munich became one of the most important musical centers in Europe. Especially the appointment of Orlando di Lasso as court musician contributed to this fame. Besides the famous stock of printed music in the 16th and 17th centuries, the extraordinary inventory of the choirbook tradition bears witness to this fact. Here, the term 'choirbook' means to represent a source with part music, showing several vocal parts on one or two opposite pages. This notation method was established long before the arrangement of polyphonic music in scores, which is still common today. The earliest manuscript in this collection (Clm 560) dates from the 12th century. 75 choir books in large folio format originate from the Court chapel, mainly from the time of Orlando di Lasso as Hofkapellmeister. Several particularly splendidly illuminated choir books originally belonged to the personal collections of the Bavarian dukes and electors. In the course of secularisation from 1802 onward, further choir book manuscripts of importance were transferred from monasteries to the court library. Due to the current process of ink corrosion, the condition of most of the stock is extremely delicate, causing some of the manuscripts to be excluded from any usage. Digitization makes the access to these sources possible again. Pre-existant black-and-white digitizations (based on the microfilm copies made in the 1960s) are kept parallel to the new, making it possible in some cases to see the elder state of the manuscripts and compare it with the actual condition.