The trombone made its first appearance in the middle of the 15th century. It is a brass instrument with a cup-shaped mouthpiece and a slide that enables the player to shorten or lengthen the tube and hence the notes of a particular harmonic series. The early trombone was known in English as a sackbut. The instrument had ceremonial associations and in the later 18th century was only occasionally used in the orchestra, notably by Mozart in his masonic opera The Magic Flute (Die Zauberflte) and in his Requiem Mass. With Beethoven the trombone becames an accepted if not indispensable part of the orchestra.