A trumpet becomes a closed tube when the player presses it to the lips; therefore, the instrument only naturally produces every other overtone of the harmonic series. The shape of the bell makes the missing overtones audible. Most notes in the series are slightly out of tune and modern trumpets have slide mechanisms for the first and third valves with which the player can compensate by "throwing" (extending) or retracting one or both slides, using the left thumb and ring finger for the first and third valve slides respectively.