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 them as logos for the 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.