Guitar chords in the key of Bb minor

The seven chords of Bb minor are Bbm, Cdim, Db, Ebm, Fm, Gb, Ab. Each one has a job in the key, and each tends towards particular chords next.

• Bbm, the tonic of Bb minor. Usually goes to Ebm or Fm: i moves to iv, which introduces the key’s flat 6th. Begins and ends the phrase, as the tonic of any key does -- but a cadence onto it normally arrives from a chord the natural minor has not got, since V needs a raised third to have a leading tone. So the plain triad here is usually preceded by something borrowed.
• Cdim, the supertonic of Bb minor. Usually goes to Fm: roots falling a fifth: the minor key’s ii–V. The root and ♭5 form a tritone. Because those notes usually resolve by step into the next chord, this chord is most often used as a passing or predominant chord. The minor key's approach chord, and rarely played as a bare triad: in practice it turns up as the half-diminished seventh, or in first inversion so the ♭5 is not in the bass, and either way it is on its way to the dominant.
• Db, the mediant of Bb minor. Usually goes to Gb or Ab: a fourth up inside the relative major. Used to brighten without leaving the key: this is the relative major's tonic, so a minor progression that moves here has the major sound and none of the work of modulating.
• Ebm, the subdominant of Bb minor. Usually goes to Fm: subdominant into dominant, the cadence the minor key borrows from major. The departure of a minor key, and the chord most often stolen from it: dropped into a major key at the end of a phrase it is the minor plagal cadence, which is why a great many songs in a major key have one minor chord and it is this one.
• Fm, the dominant of Bb minor. Usually goes to Bbm: the leading tone rises into the minor tonic. This chord is usually not played as it stands: a minor key that wants a cadence raises its third, and the minor v is kept for the places where the phrase should move without closing -- a modal or folk setting, or a descent from the tonic where the leading tone would sound out of style.
• Gb, the submediant of Bb minor. Usually goes to Ab or Ebm: the Aeolian cadence: ♭VI–♭VII–i closes the key with no dominant at all. Used at the two moments a minor key needs to sound large: as the landing of a deceptive cadence, and as the first chord of ♭VI–♭VII–i, which closes the key without a dominant and so without a leading tone.
• Ab, the subtonic of Bb minor. Usually goes to Bbm or Db: ♭VII rises a whole step to i, so there is no leading-tone resolution. Used to close a minor key without a leading tone (♭VI–♭VII–i), and as the way out of it: this is the dominant of the relative major, so a minor verse can pivot on it into a major chorus.