Uncle Bob's Guitar Lessons
The chord structure also helps you determine what major scale, or "key", a song is in by looking at the respective minor or major chords.
For example, using the chords found on the Chords Listed by Major Scale section, if you picked up a sheet of music and saw the chords F major, C major, and G major, you would know that the song is in the key of C major because that is the only key based on a major scale that will have those three chords, and in this case those chords are the IV, I, and V chords, respectively.
Fairly often you can find the key of a song simply by finding the IV and V chords, because only one key based on major scale will have two consecutive major chords.
For example, using the chords found on the Chords Listed by Major Scale section, if you picked up a sheet of music and saw the chords D major and E major, from that alone you could deduce that the song is probably in A Major because that is the only key based on a major scale that will have those two chords consecutively.
The information in the Determining the Major Key of a Song section is especially helpful when learning new songs as well, because knowing which chords belong in which key helps you "fill in the blanks" when learning new music.
For example, if you were playing along with a CD that features a song that you're trying to learn, and you figured that the song contained a G major and A major chord, you could deduce that the song is in the key of D major, and therefore the only other major chord that you will normally find in the song will be a D major. You can also assume that the only minor chords that you will normally find in the song will be E minor, F# minor, and B minor. Usually this should give you enough building blocks to learn the entire song.
There are, of course, exceptions to this method, such as changing keys within a song. When that happens, you simply reapply the same technique to the part of the song that follows the key change.
Note: You should also take a look at the Common Chord Progressions section for some ideas on specific patterns of chords that are used in songs.