A key (aka key signature) is a sequence of notes that sound good together. When you use a scale, say, C major, you are playing in the ‘key of C’. Writing songs in a key gives you a blueprint of the notes and chords that will generally sound good together.
Here, we’ve written some guitar scale shapes for common keys for you to write riffs and melodies with. If you play the major scales starting from the sixth note you get a darker, moodier scale known as the relative minor – so A minor is C major’s relative minor. These minor scales are every bit as common as their relative major counterparts.