Three marks here change the chord itself rather than where it lands, and one of them reaches outside the key entirely. This chart is in three beats to the bar rather than four, which changes nothing about how the numbers work and is worth meeting once so it does not surprise you later.
The minus makes a chord minor. The slash puts a specific degree in the bass and is addressed to exactly one player. The flat lowers a degree to a root the key's own scale does not contain — the chord that shows up from nowhere in a great many records and still has a number like everything else.
Below, the same eight bars in C and then in F — a fourth up, the single most common move a session makes to a written page. The middle column is what is actually printed, and it is the column that does not change.
| Bar | Written | In C | In F |
|---|---|---|---|
| 1 | 1 | C | F |
| 2 | 1/3 | C/E | F/A |
| 3 | 4 | F | A# |
| 4 | 6- | Am | Dm |
| 5 | 2-7 | Dm7 | Gm7 |
| 6 | b7 | A# | D# |
| 7 | 5sus 5 | Gsus4 G | Csus4 C |
| 8 | ◇1 | C | F |
The one mark written for a single player
Bar two is a 1 with a 3 underneath it. The chord has not changed — it is still the tonic — and for everybody except the bass player nothing about that bar is different from bar one. The slash is a bass instruction, and it is on the page because a bass player left to their own devices would have played the root, which is not what the arrangement wants.
Notice that the bass note is itself a degree. Not a letter, not a fret, not an interval below the chord: a number in the same key as everything else, so it transposes for free along with the rest of the page. That consistency is not decoration — a slash written as a letter would be the one mark on the chart that broke when the key changed.
A root the key does not contain
Bar six flattens the seventh degree. In this chart's home key that lands on a chord the key signature has no business supplying, and it arrives with no preparation and no apology, which is roughly how it works on a record too.
The realization column will spell that root with a sharp, because the shared network note table has no flats in it. A lead sheet would spell it the other way and mean the same pitch. That disagreement is a fact about letters, not about music, and it is the neatest possible advertisement for a notation that never prints a letter at all — the legend spells the whole argument out.