Because there are many higher and lower versions of notes in music, it is sometimes necessary to write notes above or below a staff. This is done using short ledger lines above or below either a treble or bass staff.
There is no restriction in the number of ledger lines that can be used; the note sequence simply continues from the highest or lowest note on the staff, depending on whether the music goes up or down.
The most common use of a ledger line is for the note Middle C, which falls between the treble and bass staves. Middle C is also at the note at center of a piano keyboard. It is a reference note against which higher and lower notes can be measured. This note is shown next, along with other notes using ledger lines.