This is from the ABRSM Grade 8 figured basses.
What does the 5/2 mean? I have not come across this before.
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Remember that figured bass tells you the intervals up above the bass pitch. With that in mind, the
5/2 above the B♭ suggests C (that's the
2) and F (that's the
Unfortunately for us,
B♭ C F isn't a chord. But notice that on beat 4 that B♭ resolves down to an A, and the figures there create
A C E♭ F, a clear V65 in the key of B♭.
When we look at this from a standpoint of counterpoint, we realize that the
C F stays the same between beats 3 and 4; we add in the chordal-seventh
E♭, but otherwise the only pitch that changes is B♭. This tells us that the B♭ on beat 3 is actually a non-chord tone delaying the appearance of A, the true chord tone. This B♭ is thus a 2–3 suspension in the bass.
2–3 suspensions are relatively common in this repertoire, so you should definitely stick this one in your memory bank!
If (and when) you encounter other strange figures in the future, see if you can explain it by means of non-chord tones; that's typically what will be taking place.