Small buttons placed in a row between the manuals. When pressed by the organist, the piston (which is effectively an electric switch) sends an electrical signal to the combination action instructing it to put on / take off those stops set for that piston. Conventionally, the pistons operate the stops of the manual above them (see also ‘Generals’).
Sometimes referred to as ‘Thumb Pistons’ to differentiate them from Toe Pistons.
Thumb Pistons may have a second touch, most frequently used to bring on pedal combinations.