Sometimes the musical staff is not large enough for the notes we want to represent. It is in such cases that we must use ledger lines to extend the scope of the staff. Ledger lines are small lines made through the pitches when the pitches are too high or too low to represent with a normal musical staff(Blatter, p. 5). Of course, although the function of the ledger line is to extend the scope of the musical staff, sometimes even though are not sufficient. There are quite a few pieces of music which use pitches that are either so or so low that ordinary ledger lines are unable to represent them(Blatter, p. 5). We will not review this special notation here though.
Blatter, Alfred (2007). Revisiting Musical Theory. Routledge, Taylor and Francis Group, NY.