Two different problems wear one word
The classical avoid note is a statement about voicing. A scale tone sits a half step above a chord tone; held inside the chord it will unsettle it; so the advice is not to hold it there. It is permissible melodically, and it is played constantly — as a passing tone, as a neighbour, as anything that moves. It is a caution about where a note may rest, and it is precise about what it detects.
It detects one thing only: a semitone rubbing upward against a chord tone. Which is why B♭ over Cmaj7 escapes it entirely. B♭ is a whole step under the major seventh, not a semitone above anything the chord contains — the note directly below it is A, and A is not a chord tone of Cmaj7. There is no friction to find. The apparatus, working exactly as designed, reports nothing.
And yet no player hears B♭ over that chord as a colour. B♭ is the flat seventh. It says C7 — a different chord, a different function, a different place in the tune. The tone did not clash with the chord. It replaced it.
That second problem needs a name of its own, because the tradition never gave it one. Call it a quality contradiction: a tone that alters a degree the chord had committed to, and in doing so asserts a different chord. Naming it is most of the work. Once the two problems are separated, each becomes simple; while they share a word, neither can be stated cleanly.
What a chord commits to
A chord symbol is a promise about some of its degrees and silence about the rest. The test for contradiction is run per degree, against what the symbol has actually promised — not against a fixed list of tones declared important in advance.
The third and the seventh carry a chord's identity. Change the third and major becomes minor; change the seventh and a major seventh becomes a dominant. These are the degrees that answer the question which chord is this.
The perfect fifth is the opposite, and its ordinariness is the clue. It appears in major, minor, dominant, suspended and sixth chords alike, unchanged in every one of them. A pianist drops it without hesitation, and nobody hears a different chord — because the fifth was carrying no quality information to lose. That is precisely why it is the omissible tone.
Three sources of commitment. A chord commits to a degree when:
— it is the 3rd, always;
— it is the 7th or the 6th, whenever the symbol states one;
— it is any degree the symbol has itself altered — the ♯5 of an augmented chord, the ♭5 of a diminished or half-diminished, the ♭♭7 of a full diminished seventh, the suspended 4th or 2nd.
The natural perfect fifth is never committed. A tone that contradicts a committed degree is OUTSIDE.
The third clause is what makes the test per-degree rather than per-tone. A fifth is uncommitted on an ordinary chord and committed on an altered one, and the difference is written on the symbol's face. Nothing about the degree itself decides this; the chord decides it, one chord at a time.
The two hard cases
Two collisions test the formulation harder than the rest, and both are answered the same way — by asking what the symbol committed to.
G♮ against Caug
The chord is C–E–G♯. Sound a natural G against it and the fifth is back where an ordinary chord keeps it — but this chord is not ordinary. The raised fifth is the only thing that makes an augmented chord augmented. Take it away and nothing distinguishes the remainder from plain C major.
So on that chord the fifth is promoted into the identity, and G♮ is OUTSIDE. The chord altered a degree; the alteration became the chord's name; a tone that undoes it undoes the chord.
E♮ against Cm7
This one is simpler and it stays simple no matter how it is approached. The 3rd is always committed. A natural third over a minor seventh chord is not a bright colour on a minor chord — it is a major third, and the chord it belongs to is a different chord.
OUTSIDE, and without exception. Whatever else a symbol leaves open, it never leaves the third open.
The same reasoning runs in parallel through the augmented and diminished families. Raise the fifth of a major chord and it becomes augmented; lower the fifth of a minor chord and it becomes diminished. In both cases the altered fifth defines the chord type rather than decorating it — which is why, on those chords, it is a degree the symbol has committed to and the natural fifth is the foreign tone.
The sus chords, where the tradition nearly writes it down
There is one place where the published literature already states a degree-specific rule, and it is worth reading closely, because it is the same idea reaching for words it does not have.
Over a 7sus4, the recommended scale is Mixolydian — and then the teaching adds: no third. The 3rd is written out of the scale that was just recommended for the chord. It is not marked as a note to handle carefully; it is removed, because the chord's whole character is the suspension, and a natural third dissolves it. That is not a friction problem. It is an identity problem, stated exactly.
But notice the tension in how it must be said. The tradition recommends a scale and then instructs you not to play one of its notes, and the word it uses for that instruction is the same word it uses for the F over Cmaj7 — a note you may pass through freely. One word covering both a passing caution and an outright removal is a word doing two jobs. The sus case is the clearest published evidence that a second concept was needed and never arrived.
The blues, which is the proof and not the exception
A flat third over a major chord contradicts a committed degree. It is also one of the most deliberate, expressive and universal gestures in the music. Both of those are true, and the second does not weaken the first — it is what the first predicts.
Look at how the blue third is actually played. It is bent. Slid into, inflected upward toward the natural third, worried at, left hanging between the two. Guitarists reach it by pushing the string; singers arrive at it from underneath. It is a gesture, not a member of a set — and the blue third isn't really major or minor. It sits between, and that between-ness is the whole point of it.
This does not break the formulation. It demonstrates it. The tone is loaded precisely because it contradicts. If the flat third over a major chord were merely another available colour, the bend would have nothing to lean on and the gesture would carry no charge at all. The expressive weight comes from the fact that the note is asserting the other chord while the harmony insists on this one, and the player holds both at once.
The same is true of the passing augmented chord, and of every sideslip that resolves. A tone that contradicts is a tone with somewhere to go. Marking it says where the tension lives; it does not say to avoid the tension. A measurement that told a player to stay away from the blue third would have measured correctly and concluded absurdly — which is the failure this whole formulation is built to refuse.
This is a diagnosis, never a verdict. The distinction is the point of writing any of it down: what is this note doing is a question a tool can help answer, and should you play it is not a question a tool is entitled to ask.
Three roles for a foreign tone
Once commitment is measured per degree, every tone outside the chord falls into one of three roles — and only one of them changes what the chord is.
| Role | What it is | What it does to the verdict |
|---|---|---|
| Extension | A foreign tone on an uncommitted degree, consonant with what the chord has stated — the 9, the 11, the 13, a ♯11 with the fifth intact, the 6 on a plain triad. | Nothing. No mark; ordinary colour. |
| Clash | The classical avoid note: a semitone above a chord tone, on an uncommitted degree. | An annotation only — handle with care. It never changes the verdict. |
| Contradiction | The alteration of a committed degree. | Outside — this is what makes a collection assert a different chord. |
The middle row is the classical concept, kept intact and kept in its place. It describes a real thing about voicing, and it is worth showing a player. It simply was never the thing that decides which chord you are hearing.
The twenty-one four-note chords
Seven triad qualities across three seventh tiers — the 6th, the ♭7 and the ♮7 — give the twenty-one four-note chords String Logic builds. Below, each one is put through the formulation with C as the root throughout. The commitments come from the symbol; the contradictions follow from them.
| Chord | Tones (on C) | Committed | Ordinary extensions | ⚠ Handle with care | Contradicts (⇒ OUTSIDE) |
|---|---|---|---|---|---|
| C6 | C E G A | 3rd, 6th | 9, ♯11 (with the 5th) | F (♮11), B♭ (♭7 — turns it into C7(13)) | E♭ (♭3), A♭ (♭13) |
| C7 | C E G B♭ | 3rd, ♭7 | 9, 13, ♯11 (with the 5th); ♭9, ♯9, ♭13 as alterations of uncommitted degrees | F (♮11), A (13 — also reads as an alteration of the ♭7) | E♭ (♭3), B (♮7) |
| CΔ7 | C E G B | 3rd, ♮7 | 9, 13, ♯11 (with the 5th) | F (♮11) | E♭ (♭3), B♭ (♭7) |
| Cm6 | C E♭ G A | ♭3, 6th | 9, 11 | B♭ (♭7 — Cm6 against Cm7) | E (♮3), A♭ (♭13) |
| Cm7 | C E♭ G B♭ | ♭3, ♭7 | 9, 11, 13 | A (13 — the Dorian 6th, also an alteration of the ♭7) | E (♮3), B (♮7) |
| CmΔ7 | C E♭ G B | ♭3, ♮7 | 9, 11, 13 | — | E (♮3), B♭ (♭7) |
| C°7 | C E♭ G♭ A | ♭3, ♭5, ♭♭7 | 9, 11, ♭13 | D (♮9), F (♮11) | E (♮3), G (♮5), B♭ (♭7), A♭ (♭13, against the ♭♭7) |
| Cø | C E♭ G♭ B♭ | ♭3, ♭5, ♭7 | 9, 11, ♭13 | D (♮9), F (♮11) | E (♮3), G (♮5), A (♭♭7), B (♮7) |
| C°Δ7 | C E♭ G♭ B | ♭3, ♭5, ♮7 | 9, 11 | D, F | E (♮3), G (♮5), B♭ (♭7) |
| C+(6) | C E G♯ A | 3rd, ♯5, 6th | 9 | F (♮11) | E♭ (♭3), G (♮5), B♭ (♭7) |
| C7♯5 | C E G♯ B♭ | 3rd, ♯5, ♭7 | 9, ♭9, ♯9 | F (♮11) | E♭ (♭3), G (♮5), A (13 / ♭♭7, against the ♯5), B (♮7) |
| CΔ7♯5 | C E G♯ B | 3rd, ♯5, ♮7 | 9, ♯11 | F | E♭ (♭3), G (♮5), A (against the ♯5), B♭ (♭7) |
| C6sus4 | C F G A | sus 4th, 6th | 9 | F♯ (♯11 — but also a semitone from the committed 4th), B♭ (♭7) | E (♮3), A♭ (♭13) |
| C7sus4 | C F G B♭ | sus 4th, ♭7 | 9, 13 | F♯, A (13) | E (♮3), B (♮7) |
| CΔ7sus4 | C F G B | sus 4th, ♮7 | 9, 13 | F♯ | E (♮3), B♭ (♭7) |
| C6(♭5) | C E G♭ A | 3rd, ♭5, 6th | 9 | F (♮11), B♭ (♭7) | E♭ (♭3), G (♮5), A♭ (♭13) |
| C7♭5 | C E G♭ B♭ | 3rd, ♭5, ♭7 | 9, ♭9, ♯9, ♭13 | F, A (13) | E♭ (♭3), G (♮5), B (♮7) |
| CΔ7♭5 | C E G♭ B | 3rd, ♭5, ♮7 | 9, 13 | F | E♭ (♭3), G (♮5), B♭ (♭7) |
| C6sus2 | C D G A | sus 2nd, 6th | 11 | B♭ (♭7); and E (♮3) — see below | D♭ (♭9), E♭ (♯9, against the sus 2nd), A♭ (♭13) |
| C7sus2 | C D G B♭ | sus 2nd, ♭7 | 11, 13 | A (13); E (♮3) | D♭ (♭9), E♭, B (♮7) |
| CΔ7sus2 | C D G B | sus 2nd, ♮7 | 11, 13 | E (♮3) | D♭ (♭9), E♭, B♭ (♭7) |
The ⚠ column is where the formulation and ordinary practice pull against each other — the places the formulation is still being tested. Those entries are working notes, not settled theory; the section below says what each of them is waiting on.
One row deserves a second look on its own account. C+(6) contains, in its own 6th, a tone sitting a semitone away from its own ♯5 — A against G♯. A chord that trips the friction rule using nothing but the notes it is made of is not a bug in the chord. It is a signal that a rule stated purely in semitones is measuring something other than what it meant to.
Where this is still being tested
The formulation above is held against cases rather than assumed, and several of those cases are not settled. They are listed here as they stand, because reasoning is more useful arguable than finished.
The upward reach from the 3rd
Stated as “a semitone either side of a committed degree”, the rule catches the natural 11 over a major chord — the F over Cmaj7. But that F is the classical avoid note, a matter of voicing and nothing more, and calling it a contradiction would say the major scale asserts a different chord than the major seventh, which is plainly false. The formulation over-reaches upward from the third, and the honest reading is that only the alteration that renames the chord should count. That is the single largest open question on the page.
Whether the 6th commits the way a 7th does
The 6th and the ♭7 are a semitone apart, so any chord that commits to one has the other sitting immediately beside it. That is why the ♭7 appears in the care column of nearly every sixth chord, and the 13 in the care column of nearly every seventh chord. Whether the 6th really carries identity the way a seventh does, or whether it is a softer commitment of some other kind, is genuinely open — and the whole 6-tier of the table depends on the answer.
The augmented sixth chord trips its own rule
C+(6) contains a tone a semitone from its own ♯5, using only its own members. A chord cannot be in conflict with itself, so either the rule is stated at the wrong grain, or this chord is a legitimate exception, or the semitone framing is the wrong framing altogether. Currently it stands as evidence for the third.
On a ♭5 chord the carve-out inverts
The ♯11-or-♭5 test asks whether the perfect fifth survives. On a chord whose fifth is already flatted there is no perfect fifth to retain, so the tritone reads as the altered fifth — and then the natural fifth becomes the foreign tone. The logic follows cleanly, but it is worth saying out loud how strange the result sounds when stated plainly: on these chords, G is the outsider over C.
Over a sus2 the rule under-reaches
A natural third dissolves a sus2 exactly as it dissolves a sus4 — the suspension is the chord. But the ♮3 is two semitones from the suspended 2nd, so a rule stated in semitones never sees it at all. That is why E appears in the care column of all three sus2 rows rather than the contradiction column, and it is an artefact of the phrasing rather than a musical judgement. The sus2 case needs its own statement.
One pitch, two meanings
The ♭♭7 of a fully diminished seventh is the same pitch as the 6th of every other quality. Whether that is a spelling detail or a real ambiguity in the test depends on the answer to the 6th question above, which is why the two are being weighed together.
None of these is presented as a defect. A formulation that survives easy cases and breaks on hard ones has told you exactly where to look next, which is the most a formulation can do. They are listed rather than smoothed over because the smoothing is what would make it untrustworthy.
Questions
No. It is a note that asserts a different chord — and that assertion is often exactly the effect a player wants. The blue third over a major chord is OUTSIDE by this measure and it is also one of the most expressive gestures in the music; it carries weight because it contradicts. The mark is a diagnosis of what a note is doing, never a verdict on whether to play it.
They are two different problems. The avoid note detects a semitone rubbing upward against a chord tone and advises against holding it in the voicing — a matter of friction. A quality contradiction alters a degree the chord committed to and asserts a different chord — a matter of identity. B♭ over Cmaj7 shows the gap: it has no friction at all, and it changes the chord.
Because it appears unchanged in major, minor, dominant, suspended and sixth chords alike, so it carries no information about which of those you are hearing. That is why it is the tone players drop first from a voicing without anyone noticing a change of chord. The exception proves the rule: on an augmented or diminished chord the fifth has been altered by the symbol, the alteration is what names the chord, and there the fifth is fully committed.
It depends on whether the collection still has the perfect fifth. Under Lydian — C D E F♯ G A B — the G is present, so the F♯ is a tension sounding over an intact chord: a ♯11. Under the altered scale there is no G at all, so the G♭ has taken the fifth's seat: a ♭5. The spelling was always encoding this; testing the actual collection is the same statement made checkable.
The general idea belongs to the tradition — players have always known that some foreign tones colour a chord and others replace it. What is the app's own is the formulation: commitment measured per degree, per chord symbol, with the fifth uncommitted unless the symbol altered it. Several cases in it are open, and they are listed above rather than hidden.
Sources
The general theory belongs to the field. The formulation set out on this page — commitment measured per degree against what a chord symbol has stated, and contradiction separated from friction — is the app's own, and it is a formalisation of something the field practises constantly and rarely writes down. Where a reference below is a forum or practitioner discussion rather than published scholarship, it is marked as such.
- “Fifth (chord)” — the standard statement that the perfect fifth is the omissible tone of a chord, and why. Reference article. Wikipedia
- “Avoid note” — the classical concept as it is standardly summarised, including the Berklee definition it reproduces. Reference article. Wikipedia
- The 7sus4 chord and its scale — the published statement of Mixolydian-without-the-third over a suspended dominant: the clearest place the literature already applies a degree-specific rule. Matt Otto, Lesson 49 · Jazz Library, suspended chords
- Blues scales and the major third — on the flat third sounded against a major chord, and on the bend as the way it is actually played. jazzguitar.be, blues scales · TalkBass thread (forum / practitioner discussion)
- Ethan Hein, “Blue notes” — on the blue third as a region between major and minor rather than a fixed scale degree. essay
- ♯11 against ♭5 on dominant chords — the working distinction as players argue it out, including the role of the surviving perfect fifth. jazzguitar.be forum, ♯11 vs ♭5 · jazzguitar.be forum, ♭5 dominants · DjangoBooks forum (all three: forum / practitioner discussion)
- Ari's Bass Blog, “♭5” — a practitioner's account of when the flatted fifth is an alteration of the chord rather than a tension above it. post (practitioner writing)
- Dmitri Tymoczko, “The Geometry of Musical Chords” — chord identity and voice-leading distance treated geometrically; consulted for the shape of the idea that some moves preserve a chord and others relocate it. paper
The companion to this page is The Avoid Note, which takes the friction half of the same law — what a note does inside a voicing. Between them they cover the two problems that were sharing one word.