The problem of reading intervals on a diatonic staff
There are two issues with reading intervals on diatonic staves which are used by both Traditional Notation and WYSIWYP. However, thanks to SNapp, these issues can be mitigated with the color-coding of chord intervals for the latter. The issues involve (1) the inconsistent semitone differences between the seven diatonic degrees, and (2) the use of key signatures.
The gaps - the cause of inconsistent staves
Reading chord intervals on Traditional Notation (TN) sheet music is not straightforward. With a diatonic staff, the intervals between adjacent degrees (the naturals) are not consistent. Most of them are two semitones apart, but at the B-C and E-F boundaries they are only one semitone apart. I like to call these boundaries “gaps” because as seen on the keyboard, there are gaps in the row of black keys at these points. As a result, chords that have the same “look” can have different intervals between them.
Take this example of chords E-G and F-A. While these two chords have the same visual “pattern” of noteheads (“stacked up straight” and “touching”), the 1st interval is three semitones and the 2nd is four (as labelled underneath). All because the first one crosses the E-F gap and the second one does not.
The Alternative Notation WYSIWYP was designed to make reading music as logical and as intuitive as possible. Thus, the primary audience includes beginning students of music, and keyboard players in particular. The WYSIWYP diatonic staff helps ease the Traditional Notation problem by placing staff lines on degrees C and F (red and blue lines respectively) so that the gaps are more easily seen. See a more complete summary of the WYSIWYP design here.
A notehead touching a line from below represents the degree below the gap (on the keyboard, it is the key to the left of the gap). Because the noteheads of the naturals overlap by 50% on the staff, a notehead centered on a C or F staff line represents the degree above the gap (keyboard key to the right). Thus, the E-G chord crosses the E-F gap and has an interval distance of three semitones. But the F-A chord does not cross a gap and has an interval of four.
The staff lines help to see the gaps, but reading the interval still requires careful attention to the relationships of noteheads and staff lines. In any case, without even thinking about the interval, the musician can play these chords simply by reading and playing the two notes individually.
key signatures
Reading intervals between notes in Traditional Notation is further complicated by key signatures wherein either note could be implicitly changed to represent a sharp or flat. In the following example, measure 1 is the same as above with a key signature of C Major. In measure 2, the key signature has been changed to E flat Major and so the chords now are Eb-G and F-Ab . This causes the intervals to be reversed from measure 1, even though the notehead patterns look the same.
With WYSIWYP notation, the noteheads explicitly define naturals and sharp/flat combinations so that there is no need to remember a key signature. In the figure, circles are naturals while rectangles are sharp/flat combinations. (By the way, these rectangles look like the ones above that are TN rests. WYSIWYP has no rest symbols though, just white space.)
Explicit noteheads help identify the individual notes regardless of key signature, but they don’t necessarily help in easily identifying the intervals between them.
With two-note chords, recognizing intervals may not be so important. Where this approach has some potential value though is with triads. To read and play these, one must consider three notes and two intervals. And with the most common triads, it is guaranteed that at least one of the two intervals is going to cross a gap. This doubles the challenges (gaps and key signatures) of reading two-note chords (for a further description of the problem, see https://musicnotation.org/tutorials/reading-playing-music-intervals/).