Some systems have two lines per octave, evenly spaced at a half an octave apart (one tritone or six half-steps). Bold, dashed, or ledger lines are used to allow identification of lines when multiple staves are stacked contiguously. One system uses only one line per octave, giving it the least dense appearance of any system, but it also uses the most ledger lines, with 7 of the 12 pitches requiring them. Some systems use hollow or solid noteheads or shaped noteheads to help differentiate between adjacent pitches.  (Also see Equiton, which has one line per octave but lacks pitch-proportionality.)