The 12 Bar Blues is a pattern of chords which repeats every 12 bars. It is the most common progression in Blues music, and is used extensively in Rock and Jazz as well. There are usually three different chords used. Each of these chords relates to one basic key. As mentioned in the previous lesson, the key is the central note around which a piece of music is based.
For example, a piece of music in the key of C would often be based on notes from the C major scale. Chords can be built from any notes of the scale. In Blues, the most common chords are those built on notes 1, 4 and 5 of the major scale. So in the key of C, these chords would be C, F and G, as is shown in the following diagram. For more detail about major scales in general, see Progressive Complete Learn to Play Guitar Manual.
Chord numbers in music are usually indicated by Roman numerals, as shown here.