Answer: Dmin

The chords are:

Dmin
Bb2
C2

Not all progressions are this easy to solve but this progression is relativity simple because it is diatonic. It is the relationship between major and minor chords which provides the clues.

The only place where two major chords are a tone apart is when they are the IV and V chords. The two major chords are Bb and C which means that the I chord is F. The I chord determines the key for any piece of music. This doesn't mean that the first chord determines the key as the I chord and the first chord can be two different things. In this example the first chord is Dmin but the I chord is F.

Even though the I chord is F the piece would be considered to be in Dmin. See the section on complimentary intervals for an explanation of the relationship between the I chord and the relative minor.

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