2.16Key signatures are compressed instructions

When sheet music has one sharp:

F♯

you don't need to see a sharp written beside every F.

That key signature commonly means:

G major or its relative minor, E minor.

Two sharps:

F♯ C♯

commonly means D major / B minor.

Three:

F♯ C♯ G♯

A major / F♯ minor.

We'll deal with the relative minors properly in Module 3.

For now, think:

Key signature = tell me which version of the seven-note system we're using.