The Choralis Constantinus is a collection of over 375 Gregorian chant-based polyphonic motets for the proper of the mass composed by Heinrich Isaac and his pupil Ludwig Senfl. The genesis of the collection is a commission by the cathedral of Constance for Isaac, at that time the official court composer for the Holy Roman Emperor Maximilian I, to compose a set of motets for the special holy days celebrated in the diocese of Constance. Isaac was in Constance at the time (April 1508) with the Imperial court as Maximilian had called a meeting of the German nobility (Reichstag) there. The music was delivered to the Constance cathedral in late 1508 and early 1509.