A seventh chord is a normal major chord ( the 1, 3, and 5 notes) with the addition of a flatted 7th note. You can make a seventh chord by moving an extra 1 note down to a flatted 7, or you can take an extra 5 note and move it up to a flatted 7. You can even omit the 1 note and still get that unique "7th" sound, as long as there is a flatted 7 in there. Assignment is to practice all the 7th chords that are shown in the Blues materials. These chords should be committed to memory.
