29 rules

Accidentals and pitch spelling

When does an accidental have to be printed, and which spelling is correct?

Two entangled questions. The first is display: given a key signature, a bar position, a tie, a voice and an octave, must the glyph appear, must it be suppressed, or is it a courtesy the engraver may choose. The second is spelling: of two enharmonically identical pitches, which letter name is right, which is decided by key, melodic direction and harmonic function rather than by preference.

Breaks the readingCheckable from the score alone
A key signature change requires explicit cancellation naturals for every accidental removed from the old signature not present in the new one
Any sharp/flat in the OLD signature absent from the NEW one must be cancelled with an explicit natural before the new signature, in standard cancellation order (reverse of the order the accidentals were originally added, following the circle of fifths).
Breaks the readingCheckable from the score alone
A tied note across the barline keeps its accidental without restating it
A note tied across a barline shows no accidental on the continuation; the pre-barline accidental governs the rest of the new bar as if restated there.
Breaks the readingCheckable from the score alone
Accidental duration is scoped to one bar, one letter name, one octave, one voice
An accidental governs only the exact written pitch (letter name + octave) in the measure in which it appears, for the remainder of that measure, in the voice/part it was written in. It does not apply to the same letter name in a different octave, to the enharmonic respelling of the same pitch class, or to a different voice sharing the stave.
Breaks the readingCheckable from the score alone
Accidentals track independently per voice on a shared stave
When multiple voices share one stave (SATB, or Gradus's voice2/voice3/voice4), an accidental in one voice does not cancel or apply to the same letter/octave in another voice within the same bar.
Breaks the readingNeeds both
Borrowed chords (modal mixture) are spelled using the parallel minor's flat-side scale degrees
Chords borrowed from the parallel minor (♭III, ♭VI, ♭VII, iv, ii°, etc.) are spelled with the flat-side letter/accidental consistent with the parallel minor's own diatonic spelling, not the major key's own accidental-preference side.
Breaks the readingCheckable from the score alone
Cancelling a double sharp/flat all the way to natural uses a single natural sign, never two
Fully cancelling a double accidental to the unaltered pitch uses ONE natural sign, never two.
Breaks the readingCheckable from the score alone
Courtesy/cautionary accidentals are parenthesized; functional accidentals are not
When a courtesy accidental is warranted, standard practice renders it in parentheses to distinguish reminder from functioning accidental.
Breaks the readingNeeds both
Secondary dominants and their leading tones are spelled relative to the tonicized key, not the global key
A secondary dominant's chromatically-altered leading tone must be spelled as the leading tone OF THE TONICIZED KEY, not a pitch-class-convenient letter matching the global key's preference.
Breaks the readingCheckable from the score alone
The leading tone in minor is always spelled as the raised 7th scale-degree letter, never its flat-side enharmonic equivalent
In harmonic and melodic minor, the leading tone is spelled as the letter a diatonic 2nd below tonic (e.g. A minor's leading tone is G#, never Ab, since Ab is a diminished 2nd/near-unison from A).
Breaks the readingNeeds both
The Neapolitan chord is spelled as a major triad on the lowered 2nd scale degree (♭2̂), never as a sharped-1 or other respelling
The Neapolitan's root must be spelled as the ordinary 2nd-degree letter with a flat/natural, never a sharped tonic or other substitute that obscures its flat-2 identity.
Breaks the readingCheckable from the score alone
The public Notation API cannot express naturals, double accidentals, or courtesy marks
A complete accidental vocabulary requires at minimum sharp, flat, natural, double sharp, double flat, and a courtesy/cautionary flag.
Breaks the readingCheckable from the score alone
The tritone is spelled as an augmented fourth or a diminished fifth depending on its resolution direction
A tritone resolving OUTWARD (diverging toward a sixth) is spelled as an augmented fourth; one resolving INWARD (converging toward a third) is spelled as a diminished fifth -- reflecting each tone's independent voice-leading tendency. This is the specific spelling concern the commissioning brief names as needing a rigorous, implementable algorithm.
Misleads the readerNeeds the rendered page
Accidental glyph must clear the previous chord's rightmost extent
An accidental must not collide with the notehead(s), ledger lines, or dot glyphs of the immediately preceding chord/note.
Misleads the readerNeeds the rendered page
Accidental glyph must not collide with a ledger line
An accidental beside a notehead on/crossing a ledger line must not visually overlap the ledger line stroke.
Misleads the readerNeeds the rendered page
Accidental glyph must not collide with the stem of an adjacent note or chord tone
An accidental's glyph must clear stems of neighboring notes in the same/adjacent voice and of other chord tones.
Misleads the readerNeeds both
Accidentals in a chord stack in offset columns to avoid collision, nearest-interval-first
When chord notes are close enough that accidental glyphs would overlap, colliding accidentals are pushed into successive columns further from the noteheads, closest conflicts getting the widest offset.
Misleads the readerCheckable from the score alone
Ascending melodic minor raises 6 and 7; descending reverts to natural minor's 6 and 7
The 6th/7th scale degrees are raised when the line ascends stepwise through them, and revert to natural-minor spelling when descending stepwise; harmonic minor's raised 7th applies regardless of direction when functioning as a leading tone.
Misleads the readerCheckable from the score alone
Barline cancels all bar-local accidentals with no natural required
An accidental's effect ends at the next barline; the following bar's first occurrence reverts silently to key signature/natural with no cancellation natural needed.
Misleads the readerCheckable from the score alone
Cancelling a double sharp/flat to a single accidental: modern convention vs. natural-plus-accidental
Cancelling a double sharp/flat down to a single sharp/flat on the same letter, modern convention uses JUST the single accidental, no natural.
Misleads the readerCheckable from the score alone
Chromatic passing tones: ascending motion spells with sharps, descending motion spells with flats
A chromatic passing tone between two diatonic degrees a whole step apart is spelled as the sharped lower neighbor ascending, or the flatted upper neighbor descending (C4-C#4-D4 ascending; D4-Db4-C4 descending).
Misleads the readerCheckable from the score alone
Do not precede a sharp/flat with a redundant natural in modern notation (older-practice anti-pattern)
Older editions habitually insert a natural before an accidental of the opposite type even with nothing to cancel, purely for emphasis. Modern convention omits this as clutter.
Misleads the readerCheckable from the score alone
Grace-note accidentals follow the same bar/key/voice-context rules as full-size notes
A grace-note accidental is scoped and displayed by the same bar-local, key-aware, voice-scoped rules as any other note -- it is not exempt from the accidental-scope rules above.
Misleads the readerCheckable from the score alone
No way to author an accidental attached above/below an ornament symbol (trill, turn, mordent)
An accidental modifying an ornament's auxiliary pitch (e.g. a trill needing a flattened upper auxiliary, or a turn with a chromatically altered upper/lower note) is placed above or below the ornament symbol itself, not as a normal note-attached accidental.
Misleads the readerCheckable from the score alone
Redundant courtesy accidentals are clutter, not caution
Restating an accidental on every recurrence of an already-governed pitch, or adding courtesy marks at every octave/voice boundary regardless of context, is over-notation and should be removed.
Falls short of best practiceCheckable from the score alone
Baseline spelling rule: prefer the letter name diatonic to the current key
For a pitch class with two plausible spellings, prefer whichever is diatonic to the current key; if neither/both are diatonic, prefer the key's sharp/flat accidental-preference side.
Falls short of best practiceCheckable from the score alone
Courtesy accidental expected across a system/page break or after a rest interrupts an active accidental
A courtesy accidental is conventionally expected when a previously-altered pitch recurs after a system break, page turn, or a rest long enough that the reader could lose track -- even within the technically-valid same bar or into a new system's first bar.
Falls short of best practiceCheckable from the score alone
Courtesy natural expected when a diatonic-looking note contradicts the key signature after recent chromatic activity
After recent chromatic alteration of a letter, a nearby return to that letter's plain key-signature spelling is sometimes given a courtesy mark to reassure the reader, though not technically required.
Falls short of best practiceCheckable from the score alone
Quarter-tone and other microtonal accidentals are out of scope for Gradus v1
Quarter-tone and microtonal accidentals (SMuFL's full quarter-tone/Sagittal glyph sets) exist in the broader notation ecosystem but are not part of standard tonal/modal pedagogy.
Falls short of best practiceCheckable from the score alone
Same-bar accidental in a different octave: courtesy accidental convention and disagreement
Strictly, accidental scope covers only its own octave for the bar -- a sharp on F#4 does not affect F5 later in the bar. Many engravers add a courtesy accidental in the other octave anyway to prevent eye misreading across the octave break.

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A key signature change requires explicit cancellation naturals for every accidental removed from the old signature not present in the new one

A tied note across the barline keeps its accidental without restating it

Accidental duration is scoped to one bar, one letter name, one octave, one voice

Accidentals track independently per voice on a shared stave

Borrowed chords (modal mixture) are spelled using the parallel minor's flat-side scale degrees

Cancelling a double sharp/flat all the way to natural uses a single natural sign, never two

Courtesy/cautionary accidentals are parenthesized; functional accidentals are not

Secondary dominants and their leading tones are spelled relative to the tonicized key, not the global key

The leading tone in minor is always spelled as the raised 7th scale-degree letter, never its flat-side enharmonic equivalent

The Neapolitan chord is spelled as a major triad on the lowered 2nd scale degree (♭2̂), never as a sharped-1 or other respelling

The public Notation API cannot express naturals, double accidentals, or courtesy marks

The tritone is spelled as an augmented fourth or a diminished fifth depending on its resolution direction

Accidental glyph must clear the previous chord's rightmost extent

Accidental glyph must not collide with a ledger line

Accidental glyph must not collide with the stem of an adjacent note or chord tone

Accidentals in a chord stack in offset columns to avoid collision, nearest-interval-first

Ascending melodic minor raises 6 and 7; descending reverts to natural minor's 6 and 7

Barline cancels all bar-local accidentals with no natural required

Cancelling a double sharp/flat to a single accidental: modern convention vs. natural-plus-accidental

Chromatic passing tones: ascending motion spells with sharps, descending motion spells with flats

Do not precede a sharp/flat with a redundant natural in modern notation (older-practice anti-pattern)

Grace-note accidentals follow the same bar/key/voice-context rules as full-size notes

No way to author an accidental attached above/below an ornament symbol (trill, turn, mordent)

Redundant courtesy accidentals are clutter, not caution

Baseline spelling rule: prefer the letter name diatonic to the current key

Courtesy accidental expected across a system/page break or after a rest interrupts an active accidental

Courtesy natural expected when a diatonic-looking note contradicts the key signature after recent chromatic activity

Quarter-tone and other microtonal accidentals are out of scope for Gradus v1

Same-bar accidental in a different octave: courtesy accidental convention and disagreement