The four fingers of the left hand can each be used in two different positions. The key normally operated by the index finger is primarily used for E5, also serving for trills in the lower register. Its main assignment is the upper tone hole. This hole can be closed fully, or partially by rolling down the finger. This half-holing technique is used to overblow F3, G3 and G3. The middle finger typically stays on the centre hole on the tenor joint. It can also move to a lever used for E5, also a trill key. The ring finger operates, on most models, one key. Some bassoons have an alternate E key above the tone hole, predominantly for trills, but many do not. The smallest finger operates two side keys on the bass joint. The lower key is typically used for C2, but can be used for muting or flattening notes in the tenor register. The upper key is used for E2, E4, F4, F4, A4, B4, B4, C5, C5, and D5; it flattens G3 and is the standard fingering for it in many places that tune to lower Hertz levels such as A440.