Mats Norrman:

>Is Eb4 and F3 played simultaneously a dissonant accord? And if not, what
>happens if A4 or Ab4 is added?

Yes, it's a 7th.  (If you play Eb3, F3 you'll have a major 2nd., another
dissonance).  If you add A4, you'll have a major 6th (consonance) with F,
but a major fourth (dissonance) with Eb.  The result will be an incomplete
7th dominant chord (without C) in F, which falls in Bb Major (move Eb to D,
A to Bb and keep F).

If tou add Ab4 you'll have a minor 6th (consonance) with F, and a perfect
fourth (consonance) with Eb.  The result would be a sort of appoggiatura on
Db Major chord (keep Ab and F, and move Eb to Db).

Pablo Massa
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