How does a player piano work?

A pianola or player piano works on a pneumatic system where a vacuum is built up by bellows that are driven by pedals or an electric power.

A paper roll is pulled over a metal bar with a row of punch holes. In the paper roll you can find punch holes too; one little hole for each tone.

Modern types also have holes for dynamics and pedal-use. Each punch-hole is connected with an ingenious system of valves and membranes. When air sucks through the hole in the paper, it lifts a corresponding membrane, which opens a valve, which closes a little bellow.
This little bellow sets into motion the action of this particular tone and the hammer hits the string.
The speed, in which the paper roll passes the bar with the punch holes, indicates the tempo. There are special levers to adjust the tempo. The volume of the tone can be increased or decreased by the force in which the bellows are driven.

One can find special piano player rolls, played by famous artists and composers of old times. The pianola roll, when it was invented, made it possible to record complete pieces.

The only alternative in those days: the record with 78 revolutions per minute had a limited playing time.
The pianola gives us an opportunity to listen to the way a composer like Rachmaninov performed his Prélude in g minor.