I stumbled upon this very harmonically simple piece:The succession for the first part of the song is (in the key of Eb minor) (e.g. from 0'17'')
I V ? VI I and so on
I recognize that the chord for ? is "b d f" all natural. But what type of chord it actually is? Is it some kind of "augmented minor fifth"? I cannot find any indication of this on the internet or on harmony manuals.