closed lips, producing a "buzzing" sound into the mouthpiece and starting a standing wave vibration in the air column inside the trumpet. The player can select the pitch from a range of overtones or harmonics by changing the lip aperture and tension (known as the embouchure). The mouthpiece has a circular rim, which provides a comfortable environment for the lips' vibration. Directly behind the rim is the cup, which channels the air into a much smaller opening (the back bore or shank) that tapers out slightly to match the diameter of the trumpet's lead pipe. The dimensions of these parts of the mouthpiece affect the timbre or quality of sound, the ease of playability, and player comfort. Generally, the wider and deeper the cup, the darker the sound and timbre. Modern trumpets have three (or, infrequently, four) piston valves, each of which increases the length of tubing when engaged, thereby lowering the pitch. The first valve lowers the instrument's pitch by a whole step (two semitones), the second valve by a half step (one semitone), and the third valve by one and a half steps (three semitones). Having three valves provides eight possible valve combinations (including "none"), but only seven different tubing lengths, because the third valve alone gives essentially the same tubing length as the 1–2 combination. (In practice there is often a deliberately-designed slight difference between "1–2" and "3", and in that case trumpet players will select the alternative that gives the best tuning for the particular note being played.) When a fourth valve is present, as with some piccolo trumpets, it usually lowers the pitch a perfect fourth (five semitones). Used singly and in combination these valves make the instrument fully chromatic, i.e., able to play all twelve pitches of classical music. For more information about the different types of valves, see Brass instrument valves. The pitch of the trumpet can be raised or lowered by the use of the tuning slide. Pulling the slide out lowers the pitch; pushing the slide in raises it. To overcome the problems of intonation and reduce the use of the slide, Renold Schilke designed the tuning-bell trumpet. Removing the usual brace between the bell and a valve body allows the use of a sliding bell; the player may then tune the horn with the bell while leaving the slide pushed in, or nearly so, thereby improving intonation and overall response.