I was under the impression that whenever you see a mordant or a trill, the auxiliary note is the next note up in the key signature.

But I'm a bit confused on how to proceed here. This is measure 57 of the first movement of Beethoven's Eighth Piano Sonata in C Minor (the piece is in common time).


(The measure uses the treble clef). The first mordant seems simple enough: I play a G-flat, I briefly brush a whole step up to the A-flat (since A-flat comes after G in the key of C Minor), hold the G-flat for the rest of the note's duration, and then play the F in the third beat.

But what about the second mordant? What's the auxiliary note? Normally, it'd be a G-natural, since in C Minor G-natural follows the F, but that accidental on the G-flat earlier in the measure is confusing me. Does it carry over to the auxiliary note on the second mordant? For the third beat in the measure, do I play F—G-flat—F or F—G—F?

  • It would be better if you can show us the excerpt where this comes from. – Neil Meyer Sep 5 '16 at 10:25

The rules of modern notation are that once an accidental has been marked, it continues on that pitch for the rest of the measure, unless removed by a natural. So, presumably, if the editor thought the G shouldn't be flat for the second mordent, there would be a natural somewhere. (Though, admittedly, I have no idea how you would notate a natural on a mordant like that if you did want it a G natural.)

  • Thank you! I think there would be an accidental in parenthesis above the mordant to show that the accidental applies to the auxiliary. – Sergei Nikolayvich Sep 4 '16 at 23:55
  • I was raised to believe those are "courtesy accidentals" and not to be counted on to be there. – Codeswitcher Sep 5 '16 at 0:02
  • @Codeswitcher No, a courtesy accidental is one that is redundant (it repeats what would already be true, maybe because the bar is long and complicated and the effect of the standard rules would be easy to overlook). If an auxiliary note is supposed to be different from what the key signature and the context within the bar would otherwise determine, then it's not redundant and it has to be spelt out, even though it looks small and unimportant. – Kilian Foth Sep 5 '16 at 6:42
  • I have seen accidentals above and below mordents and turns to show that the upper/lower notes are different from expectation. – G-Cam Sep 6 '16 at 13:54
  • @KilianFoth Yes, that's exactly what I'm talking about. Sergei said he thought there would (should) be an accidental in parenthesis above the mordant to show that the previously noted accidental in the same bar (still) applies to the (same) pitch: it is redundant. – Codeswitcher Sep 7 '16 at 17:07

The basic principle is "Follow the harmony".

I'm looking at the first edition which doesn't have bar numbers printed, but the first occurrence of these mordents is over a perfect cadence in E flat minor. So play the "ornamental notes" as A flat and G flat to agree with that key.

The second occurrence 8 bars later is a perfect cadence in D flat major. In that case the ornamental notes are G flat (even though the flat is not explicitly marked in that bar in the right hand) and F natural.

Any accidentals appended to this type of ornament are usually editorial additions.

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