Valves are used to change the length of tubing of a brass instrument allowing the player to reach the notes of various harmonic series. Each valve pressed diverts the air stream through additional tubing, individually or in conjunction with other valves. This lengthens the vibrating air column thus lowering the fundamental tone and associated harmonic series produced by the instrument. Designs exist, although rare, in which this behaviour is reversed, i. e. , pressing a valve removes a length of tubing rather than adding one. One modern example of such an ascending valve is the Yamaha YSL-350C trombone, in which the extra valve tubing is normally engaged to pitch the instrument in B♭, and pressing the thumb lever removes a whole step to pitch the instrument in C. Valves require regular lubrication.