Functions and rules

Writing a function

A chart row may end with the chord's function, after the scale: 5 | A7 | | V7/ii. Leave the scale cell blank and the scale follows the function, at the level the sheet is set to; a scale written in the cell is played as written instead. The function is read as:

[Key:] [(] [sub] [♭|♯] NUMERAL [quality] [/ [♭|♯] NUMERAL] [)]

• The numeral counts from the row's key area, or from the key before a colon: Eb: V7/ii.
• Case follows the chord family: upper case for major, dominant and sus; lower case for minor, half-diminished and diminished. The editor corrects the case, but rejects a major quality on a lower-case numeral.
• sub (a tritone substitute) goes only on a dominant: subV7, subV7/ii.
• After the slash comes the chord it leads to, as a degree of the key: V7/ii, iiø7/vi, ♯iv°7/V.
• Degrees count from the tonic as in a major key, in minor keys too: ♭III in C minor is E♭.
• (V7/ii): a dominant that doesn’t resolve as expected; the scale is the same. The sheet prints the parentheses.
• Berklee spellings read too: II-7, VII-7(♭5), V7(sus4), SubV7/ii, I∆7 (Δ or ∆), B♭: V7/ii.

Qualities

When a function won't read

A function settles the numeral and the key the rules reason in. It doesn't change the chord: the chord symbol, its labels and the practice presets still read the chord as written. With the scale cell blank, the function decides the scale; on a dominant the rules still choose the tensions from the key, so a function is usually better than picking the scale yourself: transpose the chart, or change the level, and the scale still fits.

Functions and their scales

Each row is a one-chord chart run through the same analysis the sheets use: the chord the function puts in the key, the scale the rules give it, the rule that decided and why. Where the rules can only guess, the rule is marked (guess), and the note on the sheet says “a guess — an @key or a function would settle it”. In a real chart the chords around a chord can settle what one chord on its own leaves open.

How the engine chooses a scale

Each chord family has its own rules, tried in the order listed below; the first that applies decides. They are grouped here by what they look at, from the chord symbol alone to the chords around it.

The order each family tries

• major: M1 → M2 → M5 → M6 → M3 → M4
• dominant: D1 → D7 → D2 → D8 → D3 → D4 → D5 → D9 → D6
• minor: m1 → m2 → m3 → m6 → m10 → m4 → m5 → m9 → m7 → m8
• half-diminished: h2
• diminished: d1
• sus: s1 → s4 → s2 → s3

Dominants

A dominant's scale is named by three tension slots: the 9th (9, ♭9, or ♭9 and ♯9 together), the 11th (11 or ♯11) and the 13th (13 or ♭13). A rule that derives them (D4, D5, D8) reads each slot from the key's own scale: major in a major key, natural minor in a minor key, harmonic minor for the minor key's own V7. It takes the altered form only when the key holds it and not the natural one. Tensions written in the symbol (G7b9, 7#11, alt) fill their slots over the rule's, and the rule id says so: G7b9 going to C in C major is D1+D4, G Half-Whole Diminished. The slots name the scale:

Reporting a disagreement

The rules are a starting point, and a tune can know better. If a scale looks wrong to you, send:

• the chart row: the chord, its function and its key area;
• the scale shown;
• the rule id;
• the scale you expected, and why: a melody note, a recording, a book.

On the Changes sheet, click a chord's numeral or scale to see why it was chosen; Report issue in that note copies exactly this: the chord's bar, and a link to the chart unless it's a library chart you haven't edited. Paste it into the contact form or a new GitHub issue, and fill in what you expected.