Bsrgvty

Can someone please help me understand this chord progression that actually takes the listener from E♭ to B♭. There is a chord at the end of the phrase and I can't understand what chord it comes from. The notes are (F A♭ E♭). Let's call this chord "X".

Here is the progression so you can get this in context.

E♭, E♭, B♭, E♭, Cdim, X, B♭

It's common to omit some notes when forming a chord (for various reasons; depends on the instrument and the composer).

The aforementioned chord is a Fm7 (no5), which means that you play the notes that form the Fm7 chord (F A♭ C E♭), but you omit the 5th (C), thus getting Fm7 (no 5) or the notes F A♭ E♭.

One of the most common chord notes you're going to omit is the 5th. There is a great thread about this here (it's about omitting notes in jazz, but still it's worth a read):

Which notes are optional in jazz chords?

The X chord could be an Fm7 and that would perfectly fit in the E♭ scale with the other chords in the progression you mentioned.

Simplified explanation:

The diatonic triads in E♭ scale are: E♭ Fm Gm A♭ B♭ Cm Ddim.

The other answers are 100% correct: with a motion to E♭ major, the expected diatonic chord here is Fm7 with an omitted fifth (C).

But for the sake of completeness, you could also turn this into an F half-diminished seventh chord, or Fm7♭5. Doing so would mean that this omitted fifth would need to be a C♭. But notice that, since there's no C♭ in the key signature of E♭ (the starting point of this phrase), this is an example of what we call mode mixture. We say that this chord (and the C♭ itself) are borrowed from E♭ minor, the parallel key of E♭ major.

Perhaps also of interest: viewing this chord as a half-diminished seventh makes it enharmonic to the Tristan chord.

Two alternate progressions: E♭ -> B♭ (Just do it!). In a march, it is normal to go from tonic in the first section to sub-dominant in the Trio section; why not do the reverse? Second alternative, E♭ -> F -> B♭. These keys sit next to each other in the circle of fifths; the transitions are easy on the ear.

[Personal experience- 40 years on keyboard, arranging, etc.]

I agree with the other answers that it's probably an Fmin7 chord. One thing I'd like to point out is that, in the context you gave, this gives the chord progression E♭, E♭, B♭, E♭, Cdim, Fmin7, B♭. The last three chords plus another E♭ wrapping around to the beginning of the progression are Cdim, Fmin7, B♭, E♭ - which is walking along the circle of fifths before resolving to E♭. This is a very common chord progression. So the interpretation of the chord as Fmin7 is especially likely to be correct!

One possible voicing/inversion of of F A♭ E♭ gives rise to a B♭11 with an omitted root and 3rd.

That is to say, F is the fifth, A♭ is the minor 7th, E♭ is the 11th.

Cdim, B11, B♭ is plausible.