ChordLoop is a php loop that repeats until you get a four chord progression.
If you're familiar with music theory, you may already know that there are 7 chords for every key that sound great together. These are called the diatonic chords and songwriters have been using them to write hits since the dawn of popular music (and before). In the key of C, those chords go like so-
C, Dm, Em, F, G, Am and B diminished (that last one is less common).
Try putting those chords together in any random order and bam, you've got a hit. Go ahead! Try it!
Once you know about these chords, it's hard to get away from them. You hear them everywhere you go and if you're trying to write a song, you always feel like it's been done a thousand times before. To that end there are a couple of other families of chords. One is called secondary dominant chords. If we stay in the key of C, they go like this -
C7 (usually played just before an F), A (usually played just before a Dm), B (before an Em), D (before a G) and E (before an Am).
Unlike the diatonic chords, they don't always sound great unless you play them in the right order. C, G, E, Am sounds great but C, G, E, Em isn't that awesome. If you don't know the theory, you can just use your ears to decide what you like and what you don't.
Another family of chords is called mode mixture chords. Without getting into too much detail about why and how, they are as follows -
Cm, Eb, Fm, Ab and Bb
Like secondary dominant chords, these don't always sound great unless you put them in a good order. For example - C, Am, Ab, G. Fun right?
All together you have 17 chords. If you were to do an analysis of all of your favorite songs, you would find that almost every chord that gets used falls into one of these categories. If you'd like to mess around with these chords in other keys you can take a couple of semesters of music theory or you can use this handy chart from NYC Guitar School (just use the transpose button in the drop down menu to see the chords in other keys).
This light version of my iPhone app writes chord progressions with some limited guidelines. Enjoy and stay tuned for updates!
C Ab C7 F