Lines a Whole Step and Major 3rd Apart, 4 per Octave
In these systems the lines are either a whole step or a major third apart. The bold lines help the eye discriminate between different lines, facilitating pitch identification. An alternating 6-6 notehead ‘color’ (hollow or solid noteheads) makes it even easier to differentiate between pitches. See the Using Notehead Color for Pitch for more about this strategy and how it affects rhythmic notation.