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.