On acoustic and electric pianos, the balance rail is a horizontal wooden rail that sits underneath the centre point of the keys. It houses the balance rail pins, metal pins that the keys are inserted into. The balance rail acts as a pivot point as the musician presses the key – making it a critical component in the keyboard assembly.
The balance rail is attached and raised above the key bed. It is aligned parallel to the keyboard, and then correctly placed will allow for ideal pivoting of the key, with the key end (closest to musician) raised at rest position. This allows for the key to be pressed, pivoted through the balance rail, and then for the other end of the key to raise end engage the action mechanism.
The size of the balance rail depends on the type of piano – Rhodes and Wurlitzer electric pianos for example have a thinner and lighter balance rail than grand pianos, but the purpose is the same – to pivot the key. The balance rail must be as level as possible to allow even key height for all keys. This is finely adjusted by adding paper punchings on the balance rail pin between the rail and the key. A balance rail felt is inserted on the pin as well to eliminate clicking between the key and the balance rail.
A similar type of rail is found near the front of the key. This is called the front rail and is designed to catch the key when it is pressed via the front rail pin.
Hohner Clavinets do not have a balance rail as they are designed more like electric keyboard instruments. Instead they pivot at the end of the key, using a key spring to return the key to rest position.