E as a Nashville Chart Sees It

Count off in E and the seven numbers name E, F#, G#, A, B, C#, D# — in that order, every time, on a page that never mentions E anywhere.

Three of those need a mark and four do not. A bare number is a MAJOR chord, so 2-, 3-, 6-, 7- are what a chart writes when it wants the chord the key would have supplied anyway; a bare 2 means the writer wanted something else and meant it.

A piano player reads the whole chart at once — the instrument the numbers are usually written at, and the one that can voice a slash bass and the chord above it without asking anybody else to cover half of it. The digit is split between two hands on the same beat: whatever a slash mark puts under it goes to the left, the chord itself to the right, and a bare number leaves the left hand on the root.