From an exercise to identify the key of this passage, I thought it was D flat major, given the first note, but the answer is B flat minor. What in the passage indicates it as B flat minor?
One thing that can give you a hint is the presence of A♮, which is definitely more typical of Bb minor than Db major.
Starting note is largely useless as an indicator, plenty of melodies start on the third. In general the ending note works pretty well, as a verse tends to end on the tonic (though not even that is strictly foolproof), but here you're only given a single phrase, so you don't get that evidence.
In addition to the presence of A-natural, well explained in other posts, there is a second clue, but not as immediately evident.
A second approach
- The first hint is that the passage ends on F. This could be the third of a Db major harmony or the root of an F chord, the dominant of Bb minor.
- The second hint is the measure before, which emphasizes A and C — combined with F, this clearly outlines F major, the dominant of Bb minor.
- Working back another measure, C and Eb are emphasized, further reinforcing dominant harmony: F - A - C - Eb.
- Singing or humming the passage, if possible, provides a final confirmation.
But why bother, since the first approach is so simple
In this passage, the approach listed above is clearly more complicated, and unnecessary. However, consider a passage in which there are no A naturals.
Notice the emphasis placed on C in the two measures preceding the F, particularly the two immediately preceding it. This gives a strong indication of a V-I cadence in itself, even in the absence of the A naturals helping to outline an F chord (or provide the obviously raised seventh scale degree).
In key D♭ major, the A notes are diatonically A♭. That's reflected in the key signature, which of course is the same 5 flats for both keys.
However, minor keys often use a sharpened leading note, to make the V sound more convincing. So here, with A♭ sharpened to A♮, that leading note is the revealing factor. That piece (or at least that part of it) is in key B♭m in preference to D♭.
The fact that the piece starts with note D♭ is no indication that it should be in key D♭ - that note is the m3 of B♭m in any case, so fits.
Those of us who started our music studies, well, a few years ago, consider this a very elementary question! The difference between major and minor with the same key signature was the presence of the sharpened leading note. C major or A minor? A sprinkling of G♯ accidentals was the certain signal of A minor. Because the standard form of minor scale was the Harmonic Minor, with its sharpened leading note. How else could we have a proper major dominant triad to lead us home to the tonic? Common Practice harmony would collapse in flames!
Some more recent approaches to harmony consider the Natural Minor scale to be default and barely acknowledge the harmonic and melodic forms. . But this question is old-school. All those A♮ accidentals shout B♭ minor.