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To understand why these patterns occur in popular music, it is important to have a general grasp on the theory behind music and chords. Any song that you are likely to hear on the radio has a key, and every note and chord in the song revolves around that key. For the sake of this example, we will say that our key is C major, as this is one of the most popular keys, and one of the easiest to work with. Every note within a key is assigned a number, denoted in roman numerals. The number I, the first note in a key, is the key itself, in this case C. II is always a full step above that, in this case climbing up to Db (D flat) and then to D. So our first two notes are C, D. Now this is where the concept of major or minor comes in. If the key was minor, we would move up a half step to Eb, but since the key is major, we move up a full step to E. So our III is E. IV, a half step up from III, is F (remember, there is no such thing as Fb). This goes on for a while, and ultimately our progression is, I, II, III, IV, V, VI, VII, or in C major, C, D, E, F, G, A, B, and ending on another C in the next octave. Since in major keys, III, VI and VII are minor, the chords are C, D, Em, F, G, Am, Bm. This is the complete list of basic chords that can be used in a song, and it becomes apparent when looking at this that there is a very limited number of possible combinations. This is the reason for common chord progressions in popular music.