The Stem Lower Diameter is the diameter of the mouthpiece which is slid into the instrument. The slider provides the facility to modify the diameter of the lower part of the shank/stem. It operates over a 2 mm range, allowing the diameter to be both reduced and increased by one millimetre.

When this is adjusted, the taper of the stem is changed, as is the taper of the backbore as the wall thickness at the mouthpiece exit diameter is kept constant.