related discussion: fully keyedDWDS
Klappe - 1. bewegliche Vorrichtung, die eine Öffnung verdeckt, verschließt und leicht geöffnet werden kann: ... die Klappen der Klarinette, des Horns
Webster's 3rd unabridged:
¹key - ... 6 ... b: a lever by which a vent is opened or closed in the side of a woodwind instrument (as a clarinet or bassoon) or a valve or piston is controlled in a brass instrument (as a French horn or trumpet)
Random House unabridged:
key¹ - ... 12. (Music.) ... b. (on a woodwind instrument) a metal lever that opens and closes a vent.
key - ... 2: ... a lever depressed by the finger in playing an instrument such as the organ, piano, flute, or concertina.
Harvard Concise Dict. of Music, ed. Randel:
Key. (1) [F. touche; G. Taste; It. tasto; Sp. tecla] On keyboard instruments, any of the parts of the action that are depressed by the player's fingers [see Keyboard]. In woodwinds the term applies to comparable devices, i.e., the levers covering the side holes. [F. clef; G. Klappe; It. chiave; Sp. llave].
Klappe - ... e: (an Musikinstrumenten) key; (an einer Trompete) valve
Klappe - a: ... (an Oboe etc) key ...