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.