The tenor saxophone, like all saxophones, consists of an approximately conical tube of thin brass, a type of metal. The wider end of the tube is flared slightly to form a bell, while the narrower end is connected to a single reed mouthpiece similar to that of the clarinet. At intervals down the bore are placed between 20 and 23 tone holes; these are covered by pads which can be pressed onto the holes to form an airtight seal. There are also two small speaker holes which, when opened, disrupt the lower harmonics of the instrument and cause it to overblow into an upper register. The pads are controlled by pressing a number of keys with the fingers of the left and right hands; the left thumb controls an octave key which opens one or other of the speaker holes. The original design of tenor saxophone had a separate octave key for each speaker hole, in the manner of the bassoon; the mechanism by which the correct speaker hole is selected based on the fingering of the left hand (specifically the left ring finger) was developed soon after Sax's patent expired in 1866.