A number chart is written to be read once, at speed, by somebody who has not seen it before. Everything about the notation follows from that: it is small, it is unambiguous about the things a player cannot guess, and it says nothing whatever about the things a player can. The ten marks below are the whole vocabulary this site uses, and every one of them appears on at least one of the four charts.
Two rules are worth stating before the list, because a reader who has come from staff notation will get them wrong in the first bar. First: a bare number is a MAJOR chord. Not "the chord the key gives you on that degree" — a major chord. In C, a written 2 is D major, and a chart that wanted D minor would have written 2-. Second: the number is a degree of the key's major scale, and only that. It is not a function, not an analysis, and not a claim about where the music is going. It is a lookup with one input.
- 1 – 7 — A plain number
- The degree of the key's major scale, played as a major chord. In A the 4 is D; in Eb the same 4 is Ab. Nothing about the page changes when the key does. Changes the chord.
- - — The minus
- Minor. It is a suffix, not a case change: 6- is the minor chord on the sixth degree, and 2 without a minus is a MAJOR chord on the second, whatever the key signature would have predicted. Changes the chord.
- 7 — A seven
- Dominant seventh. Written when the seventh is part of the arrangement rather than a player's choice — most often on the 5 going home, and on a 2 or a 3 that has been made to pull somewhere. Changes the chord.
- -7 — Minus seven
- Minor seventh. The two marks stack in the order they are read: the minus makes the third, the seven adds the top. A 2-7 is the same chord a rhythm player would have reached for anyway; writing it stops the guesswork. Changes the chord.
- sus — Sus
- Suspended — the third is replaced by the fourth. Written bare because which suspension resolves and when is the player's call; the chart is saying only that the third is not what happens here. Changes the chord.
- b # — A flattened or raised degree
- A root outside the key's major scale. The spellings are conventional and worth memorising as spellings: b2, b3, #4, b6, b7. A b7 in A is G — the chord that arrives from nowhere in a hundred records and has a number like everything else. Changes the chord.
- / — The slash
- A bass note under the chord, itself written as a degree of the key. 1/3 is the tonic with the third degree underneath — a bass instruction, not a new chord, and the only mark on the page addressed to one player. Changes the chord.
- │ — The split bar
- Two chords sharing one bar. Drawn as a divided box rather than two boxes, because the bar count is what a reader is tracking — a split bar is still ONE bar, and the beats inside it are named so the two halves need not be equal. Changes the bar.
- < — The push
- Arrive early — the chord lands on the eighth before the downbeat and the beat itself is left empty. The single most common thing a reader misses, because the chord is written in the bar it belongs to harmonically and played in the bar before it. Changes the attack.
- ◇ — The diamond
- Hit it and let it ring. The band stops keeping time inside a diamond bar and waits; the count resumes on the next bar. Written round the number on paper, drawn as a leading glyph here so it survives being read aloud by a screen reader. Changes the attack.
One thing the numbers quietly sidestep
Ask two players to write the chord a semitone under the tonic of C and you may get Bb from one and A# from the other. They mean the same pitch and the argument is about letters. This site's realizations use the shared network note table, which spells every black key with a sharp, so a b7 in C is printed here as A# where a lead sheet would print Bb.
That is a limitation of the letters, and it is also the clearest possible argument for the digits: the chart writes b7, everybody plays the same chord, and nobody on the floor has to agree about the spelling at all. It is the same trade the notation makes with keys — you give up naming the thing, and in exchange the page stops needing to be rewritten.
What is deliberately not here
No roman numerals, anywhere on this site. They encode a quality in their case, which is precisely the thing this notation encodes in a suffix, so mixing the two would mean two contradictory rules for one page. No rhythmic notation beyond the push and the diamond: a chart names chords and bar counts, and the groove is the record's business. And no key signature, obviously — the absence of one is the point.