In music notation, a ledger line is a short horizontal line drawn just above or below the staff and parallel to it. The purpose of a ledger line is to indicate the pitch of a note which is outside the range of the staff, by representing a sixth (or seventh or eighth or more) line at the top or bottom of the staff.
Ledger lines look something like this, here used to indicate a pair of half notes at high C and middle C:
--O-
|
-|--
/\ |
---| /-------------------|---------------------------------
|/
---/-------------------------------------------------------
/|
-/-|/\-----------------------------------------------------
| | |
|--|--|-------------------------|--------------------------
\ | | |
--\|_/--------------------------|--------------------------
| |
\| -O--
If many ledger lines are required for more than one or two notes, an octave symbol may be used instead to make it easier to read.