In music theory, what is meant by the phrase "circle of fifths"?

Question

COF is a very common music theory term, along with similar terms such as "circle of fourths, thirds" etc.

Answer 1

The Circle of Fifths is a clock-shaped diagram that shows all the major scales that exist, and all the sharps and flats that each scale contains. Each scale is a perfect fifth higher than the scale before it.

C major is at the top of the clock shape. It's at the top because it has no sharps or flats. When we move clockwise to the next scale on the circle, we get to G major, which is a fifth higher than C major. G major has one sharp (F#).

If we go to the next major scale, we get to D major, which is a fifth higher than G and has one extra sharp. Go a fifth higher, and you reach A major, which has yet another sharp. And this pattern continues until you reach the bottom of the clock, which is C# major (seven sharps).

You could also call this diagram the "Circle of Fourths," because when you start at C major and move counterclockwise, each scale is a perfect fourth higher than the one before. Each time you move counterclockwise around the chart, you either add a flat or remove a sharp from your current scale.

The Circle of Fifths often includes minor scales as well.

Answer 2

The circle of fifths refers to the progression of key signatures and resulting sharps on the treble clef, starting with middle-C and the key of C-major which has no sharps nor flats. If one moves up a 5th interval from C, you are now on the key of G. G-major has one sharp (F#). Moving a 5th interval up, you are now on the key of D and D-major adds one additional sharp, adding to the F# a C# (note that moving up this progression, the sharps are cumulative - one is added to the current mix). Moving up a 5th from D we are now on the key of A and A-major adds a 3rd sharp such that we now have F#, C# and G#. This continues to it's conclusion. Alternately, if one moves downward from middle-C (with no sharps or flats) in a 5th interval, we are now on the key designated as F and F-major's key signature has a single flat, notably, B-flat. Moving down another 5th interval from F you land on B-flat and just as progressing up adds sharps, so progressing down in tone adds flats, so B-flat major adds an E-flat in the key signature to the B-flat already present. Notice a couple of principles. C-major is considered kind of the "baseline" key, having no flats or sharps. As one ascends in tone in 5th intervals, the key signatures are denoted by increasing "sharps". As you move down, a similar pattern exists but now key signatures add "flats". This is it's own mnemonic, upwards is sharp, downwards is flat. Notice something else - what is the interval of the sharps? The first one is F# then C# then G# then D# and so forth. Do you see an interval pattern? The sharps are added in intervals of 5ths as well. Let's progress along the flats, so we have B-flat the sole flat in the key signature for F-major our first key in the "circle of fifths" going down in tone from middle-C. Referring to the key signature (which is NOT the same as the "key"...so the "key" of F-major, denotes the scale mode, but the "key signature" is defined by the B-flat) we have B-flat, then added (going down a 5th from F-major to B-flat major) an E-flat to the key signature, etc.