That *doesn't* surprise me - it's hardly the first inconsistency I've come across in Grove. Does anyone have access to the full new edition, to see what's said there?
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Herewith the complete article on Acciaccatura from Revised New
Grove Online I think this clears up the confusion quite nicely:
A
'crushed note'. C.P.E. Bach (1753) and F.W. Marpurg (1755), who
provided the German translation Zusammenschlag, defined the
acciaccatura as a non-harmonic note played a tone or semitone below
any of the main notes in arpeggiated chords, and immediately released.
In 18th-century German sources such as C.P.E. Bach's treatise, it was
frequently indicated with an upward diagonal stroke through the stem
between the harmonic members of the chord. In melodic usage, the same
writers classed the unprepared, simultaneously struck dissonant 2nd
followed by the release of the lower note as a form of mordent. The
Italian theorists Francesco Gasparini (1708) and Francesco Geminiani
(1749) reserved the term acciaccatura for dissonances a whole tone
below the harmonic notes played during arpeggiation, but used the
terms mordente (Gasparini) or tatto (Geminiani) when the dissonant
note was a semitone below the main note. These writers were unclear
about the necessity of releasing the non-harmonic notes of whole tone
interval; but Geminiani stated that the tatto 'is performed by
touching the Key lightly, and quitting it with such a Spring as if it
was Fire'. Typical of their largely improvisatory practices, the
Italians did not notate the acciaccatura or mordente/tatto. In the
19th century, acciaccatura came to mean quick single grace notes,
usually a major or minor 2nd above the main note; these were defined
as short appoggiaturas in the 18th century by Quantz (1752) and C.P.E.
Bach. It is possible that these non-rhythmic short notes were seen as
the single-line instrumental or vocal analogues to the 'crushing'
of harmonic notes by dissonant notes available on the keyboard. This
type of acciaccatura is generally notated as a small note of
semiquaver value before the main note. The custom of writing these
semiquavers as quavers with perpendicular slashes through their flags
originated in the 18th-century shorthand notation of single
semiquavers (especially in Italian sources), whether ornamental or
structural notes: it is easier to draw the second flag across the
first rather than parallel to it. For further information see
Ornaments.
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