Curious about this as I have never heard about this rule/guideline. I hear people using the 3rd and the altered 9ths melodically all the time. One common example is “Cry me a river” lick that has been used over altered chords for well over 50 years.
I think the key is to see it more as a note collection of available colors from which to construct a melody than as a scale. Thinking of scales as ordered events is kind of akin to thinking of the alphabet as having meaning before it is rearranged to form words, or thinking of the colors on an artist’s palette as ordered before the artist decides what picture to paint.
I might have been a bit strict, but trying to avoid both thirds one after the other in the same harmony is preferable. Chromatic passing notes are not scale notes and can always be used in the melody as such.
If you look how the Spanish Phrygian is used in Spanish music (mostly flamenco), the major third is used in the chord below the melody, but in terms melody the minor third is used above the basic dominant chord. Also the fingering on guitar for the b9#9 is to play the major third on the G string and the minor third on the E string above.
In its traditional use it is a mix of two scales hMin V and Phrygian. The hMin V gives the dominant chord and the Phrygian the minor third.
Scale mixes are typically a switch of color inside a chord. It would often be possible to separate the scales and notate where the switch is, but this is often not practicable. And Spanish Phrygian (8-note) is a special case arisen from a certain culture that is not only western.
Everyone can do with notes what they want and therefor have a lot of options, but rules help to avoid dead ends, even if they can be broken if there is a good reason for it, and give orientation.
My rule for scales is no two consecutive halftone steps in scales (but in melodies chromatic passing notes are allowed). And I find this rule very helpful for scales that are the material for the chords for these scales.
It doesn’t work for Spanish Phrygian (8-note), but by separating the scales from the mix and using either or (except for the chord) it works there too.
I derive my rules from music, not by wanting a rule that is more important than reality. In some cases I might be wrong (like anybody), but I try to do my best.
I know that it doesn’t cover all music, but I try to start from the most structured and natural things going towards the less structured stuff at least as a perspective.
Looking at options is very open but less structured (can allow non-practical combinations) than a list of fitting scales. At the end it is (almost) the same thing from a different perspective.
Varying the options is like switching between the scales. If they don’t change too fast we recognize it as a switch, if it happens fast, we just recognize some disorder or uncertainty.
The highest disorder in the tempered system is the equally distributed use of notes of the chromatic scale.
By emphasizing some notes or removing some of the notes you get more order and orientation.
That’s the other end of the spectrum.
BTW, I had a look at “Cry me a River”, there are no two consecutive halftone steps in the melody on the same chord at the three sheet music versions I have. I could not even identify what you meant, Chris.
Very dimly I can remember a 9 b9 1 series of notes. But either the b9 is a passing note or the chord/scale changes from 9 to b9.
I’m sorry that I haven’t made clear that the no consecutive halftones is a scale rule only and does not apply to passing notes in the melody.
For me scale notes are the notes of a full chord. They support the harmony (some of them better than others, but all do).
Options due to a not fully specified chord make it possible to switch the inherent scale, but everybody should be aware of the used scale, otherwise there will be audible confusion or a least uncertainty due to scale note mixing.
If the chord/scale changes, the rule in general does not apply because it is not in the same scale.