ledger line

In music, notation used to indicate exceptionally high or low notes, consisting of short lines added above or below the stave. In music prior to the classical period it was common to change clefs in order to avoid using ledger lines. For example, if a part reading bass clef became too high, the composer would notate tenor clef. This practice is continued for cello and bassoon writing but not for other instruments such as the keyboard. The first known example of a ledger line occurs in an organ book of 1523.