Reference

Key signature

The sharps or flats written at the start of a staff, telling you which key the piece is in.

CAmGEmDBmAF♯mEC♯mBG♯mG♭E♭mD♭B♭mA♭FmE♭CmB♭GmFDmouter ring = major
inner ring = relative minor

Clockwise adds a sharp at a time, anticlockwise adds a flat at a time

Where it's taught