ledger line
A short horizontal line extending the staff to notate pitches above or below its five lines.
In Depth
When a note falls above or below the five-line staff, small additional lines called ledger lines are drawn through or near the note head to indicate its pitch. Middle C, for example, sits on one ledger line below the treble staff or one ledger line above the bass staff. Excessive ledger lines are difficult to read — more than three or four above or below the staff becomes impractical. Composers address this by using 8va and 8vb markings (play an octave higher/lower) or by switching clefs. The alto clef, for instance, exists partly to reduce ledger lines for the viola.
The highest note ever written for violin requires nine ledger lines above the treble staff — at that point, the performer is essentially guessing the pitch by position alone.