Double bass
The simplest type of mechanical aid is the use of wooden "fingers" that can be closed to press the string down and fret the C#, D, Eb, or E notes. This system is particularly useful for basslines that have a repeating pedal point such as a low D, because once the note is locked in place with the mechanical "finger", the lowest string then sounds a different note when it is played "open" (e.g., a low D).