One of the most common tools used in songwriting is the Key Cycle. Also referred to as the Circle of 5ths or Circle of 4ths. Chords have a natural tendency to move to a chord a 4th above or a 5th below the current chord. For example if you are playing a C Major chord, moving to an F Major chord (an interval of a 4th) would sound natural as the next chord in the progression.
Similarly moving to the G Major chord (an interval of a 5th) would also sound like a logical chord progression. Using this method of cycling around in 4ths or 5ths one can logically move from one key center to another. It depends on the type of chord you move to as to whether you have stayed within the original key or whether you have moved into another key.
If for example you chord progression goes from Bm to E7 to A Major you have moved around in 4ths, all the chords are chords from the key of a Major. (refer to Basic Major Chord Scales) If however your progression goes from Bm to E7 to Am to D7 to G Major you have now moved into the key of G Major. What is referred to as a IIm V7 I progession.
The Am D7 and G Major chords all belong to the key of G Major. We have moved away from our original key center of A Major using the Cycle of 4ths. Don't get confused with the alphabet letters around the circles, these alphabet letters represent the different Key Centers NOT Chord types. Below are the key cycles of major and minor!