The Circle of fifths has a big importance in music theory.
What is The Circle of Fifths?
Some people call it The Circle of Fourths, The Cycle of Fifths or The Cycle of Fourths..anyway.
The Circle of Fifths tells you how many sharps or flats are in a given key. Actually, there are other purposes for the "The Circle of Fifths" like chord substitution or determining the best transition or move (modulation) to another key. But we won't go into that here. You can see the circle of fifths diagram below;
The numbers on the inside of the circle show how many sharps or flats would be in the key signature for a major scale built on that note. Thus a major scale built on A will have three sharps in its key signature. Relative minors are shown in blue and you can find accidentals for each key in parenthesis.
While there are 15 Major Key Signatures, bear in mind that three keys are enharmonic;
B is the same sounding scale as Cb, F# is the same sounding scale as Gb, C# is the same sounding scale as Db.
Below you can find the order of sharps and flats. You can see that the order of sharps is circle of fifths and the order of flats is circle of fourths.
No matter you play guitar, piano or any instrument, please just memorize these seven letters in order, that's it! Now you can easly find the number of sharps and flats in a given key.
F C G D A E B
For example, you need to know G key;
Start counting from F to G (F, C, G). Then don't count G and note before G this is C; G major key just have F#.
A ===> F, C , G, D, A ====> A has F#, C#, G#
D ===> F, C, G, D =====> D has F#, C#
It is easy, isn't it?
But there is an exeption. You should remember that F has Bb
What about flats;
You need to know Ab, again count but this time B to F direction.
Count until A and one more letter (B,E,A and D) so Ab has (Bb, Eb, Ab and Db)
Here another one Bb (B, and one more letter E) Bb has (Bb and Eb)
Try to fill above circle of fifth / forth chart.