Seventh chord

You can simply learn the chords which "belong" together, or you can learn the "circle of fifths". If you know the circle of fifths, it's easy to find the chords for the blues. A seventh chord is based on the major chord (tonic, third, fifth) plus the minor seventh, which is one whole note below the octave. This means that the seventh chord of C (written C7) contains the notes C,E, G and Bb. (Not B, which is the major seventh). It "feels" as if it needs another chord to round it off, and it does in fact often lead to the chord which is based on the 7th chord's sub-dominant. So C7 leads to F, G7 leads to C. But this does not always happen, as you can often hear in the blues. Sometimes we play C7, F7, G7 C, where F7 is not followed by Bb.