I found this classical chord progression. What are the harmonic functions of these chords, especially the Adim.
1Amin Bdim Amin A Bb Adim Amin E Amin
I got it from here youtube.com/watch?v=zgBPiHTPCdg. The key here is C minor though.
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Based on the reference you provided in comments your version of the chords in the key of A minor is incorrect and also leaves out the all important bass notes/inversions:
G#o7/B Am/C A7/C# Bb/D D#o7 Am/E E7 Am
If you look at this screenshot you will see the chords in the key of Cm at the top with the analysis directly below them. The analysis is also in the lower right corner:
If you want to understand this beyond a bunch of Roman numerals, numbers and slashes, just listening to it reveals that the first 6 chords of this progression has a chromatically ascending bass line which has a dramatic building quality to it.
Another important aspect of this progression is that chords 1,3,5 and 7 are either diminished or dominant chords that resolve to chords 2,4,6 and 8. This gives an added tension-release as you go from chord to chord.
The Adim chord is known as a "common-tone" chord. Common-tone diminished chords are often used to extend a tonic chord.1 In this case, it's acting to delay the tonic chord or smooth the transition from the Bb chord.
Here is each chord by function:
Amin: Tonic Bdim (=G#dim): Leading-tone chord; expansion of tonic Adim: Tonic A: Tonic major Bb: Neapolitan; more tonic expansion Adim: C.T.dim; transition from N to tonic Amin: Tonic E: Dominant Amin: Tonic
1. For more on common-tone diminished chords, see Understanding function of diminished chord in Paganini's 4th caprice