These systems are similar to those with three lines, a major third apart
(above), only they have a ledger line (instead of a bold or dashed line)
for their third line per octave. This gives them an even less dense
appearance and allows for easy identification of the lines when staves
are stacked contiguously. Lines spaced a major third apart have three
note elevations in the spaces between the lines. Some systems use hollow
and solid noteheads to help differentiate between adjacent pitches.