Core Location: Theory and Mapping
Also included in the Nucleus.
The circle of 5ths is an organizational system for key signatures.
Roughly modeled after a clock, the circle indicates the 12 (15 including enharmonic keys) different key signatures.
Theoretically, there are an infinite number of keys, but who's going to play in A one million sharp?
For another way to remember key signatures, check the Lines of 7.
Each key signature (we can think of as a logo for tones to be used in a given piece of music) represents a Major key and a minor key. Key signatures are indicated at the beginning of a piece of music (between the clef and time signature).
A key signature indicates which family of tones will be utilized for a given piece of music.
In tonal music, it is an arrangement of sharps or flats which define pitches to be used. Any tone with a sharp or a flat is sharped or flatted. Any other tone/s not sharped or flatted will be natural/s.
Sharps & flats can be mixed in a key signature, but aren't for any of our basic keys.
Each key signature defines a diatonic scale in two modes: Major & minor. The type of minor is called Natural minor or the Aeolian mode. The Major/minor key centers are relatives. Diatonic means 'within the key or across the tones of a key center'. A diatonic scale is one which utilizes the seven pitches defined by a key signature for melody & harmony.
The natural minor can be found by beginning the Major scale on the 6th scale degree. Example: C and Am are relatives. 1-2-3-4-5-6 = C-D-E-F-G-A.
The natural minor key tones are identical to its relative Major key. They share the same tones (chords & scales), but calling a different chord the one (calling vi of the Major key the i).
Natural minor is a member (somewhat viable, but cannot fully stand on its own harmonically) of the minor key family. Yet, it truly doesn't relate (isn't a true relative harmonically) of the relative Major. For example, A minor is a modification of A Major and ultimately isn't 'born of' C Major.