I have never seen this before . The 1998 Henle Urtext edition of Brahms’ Paganini Variations suggests pianists use finger #0 at the end of measure #68.

I have before never noticed this fingering in any piece of piano music. Music notation is a malleable art, and sometimes a composer will invoke unique indications. But the zero finger? That just does not sound right. If one’s thumb is their first finger and the little finger the 5th finger (as has been established for centuries) then the 0 finger might be an Ivesian or Cowellian stroke. As such it would seem to have no place in a Brahms work.

If the Henle editions use of the 0 finger is intentional then the editors must be saying something about the fact that a finger in each hand could strike the same key. This is hardly an unusual situation in piano music. If an editor steps in at all with a suggested fingering then one or the other of the hands’ fingerings are often in parenthesis or italicized to reflect that the fingering is supplemental.

This style of hand-on-hand and hand-over-hand fingering is seen in a lot of Baroque keyboard music written for instruments with multiple keyboards but today played on a single keyboard. Any honest pianist will admit that on today’s pianos a work like Bach’s Goldberg Variations can be downright irritating to play, as it was not written for a single keyboard.

Generally speaking a pianist uses only one finger in these overlap situations, but it is a matter of personal choice. In the above example I can imagine a pianist, intent of maintaining the integroty of the counterpoint, at least creating the appearance of using both thumbs at once. Nevertheless the zero (zeroth?) finger still makes no sense, even as a mathematical error: 1/1 = 1, not 0. The only other edition of the Paganini Variations I have on hand (Kalmus) recommends the thumb (a.k.a. finger #1) in both hands. More telling, however, is that throughout the rest of the Henle Edition of this work there are several more instances where a single note is struck by both hands. In none of those other instances is a fingering of 0 used. Searching for other examples of finger #0 I browsed through several pages of Leopold Godowsky, assuming that the frequently overlapping voices in his Bach Cello Suite transcriptions would produce numerous occurrences of the same note being hit by two fingers, and thus plenty of opportunity for the 0 finger. As expected, there were plenty over instances of the same note struck by both hands, but never was the 0 finger suggested.

The zero finger shown here is probably a typo, maybe a software-generated glitch, though it is hard to imagine why any notation software (or a human editor, for that matter) would be programmed to recognize a finger #0.

Or maybe this notation is very common and I missed the memo back in high school.