In the key of C major, our I chord would then become CEGB which is called C major 7, written as Cmaj7 or C-with-a-little-triangle. If you play these notes together, there is a jazzy or slightly discordant sound - think of Girl from Ipanaema. This is characteristic of major seven chords, and comes from that B note, which is just one semitone away from being an octave higher than the root, C. The chord is made of notes I, III, V, and VII of the major scale, and the VII is often referred to as a major seventh note.

If you don't know 'Girl from Ipanaema', you might recognise the sound of major seventh chords in Erik Satie's Gymnopedie no. 1, Simon & Garfunkel's Old Friends(Bookends). Listen out for this sound when you're listening to music and see if you can recognise it.

Now look at the V chord in the key of C major. This contains the notes GBDF, and is called G dominant 7 or G7. Notice that the F is two semitones, one whole tone, away from being an octave higher than the root. This distinguishes the chord from a major seventh chord.

The interval in the I chord from the C up to the B is called a major seventh, a jump of 11 semitones. The interval in the V, or rather V7, chord from the G up to the F is only 10 semitones, and so musicians often refer to this F note as a flattened seventh.

The name 'dominant 7' comes from the names that classical theorists give to the notes of a major scale, the tonic being I, subdominant being IV, and dominant being V.

Beginners often find the distinction between major 7 and dominant 7 chords confusing, partly because a dominant 7 chord like G7 still has a major or happy sound, due to the interval of a major third from G to B. Once you have spent some time thinking about the numbers and intervals, you should try to get a feel for the different sounds of the different chords. Compare the jazzy 'Ipanaema' sound of the major 7 chord to the opening of 'Twist and Shout' where the notes making a dominant 7 chord are sung to 'Aaah'. You should also compare the sound of C major 7 (CEGB) to C7 (CEGBb), to see the difference made by that flattened seventh note.

Dominant seventh chords are commonly used in all kinds of music. Most beginning guitarists would learn chords such as A7, D7, E7, etc. in their first few months, whereas the jazzier sound of major seventh chords only crops up in more sophisticated songs.

Now let's look at the II chord in C major, extended from a triad to a seventh chord, made of the notes DFAC. Notice that this chord still has a flattened third, giving it a minor sound. It also has a seventh note that is 10, not 11, semitones higher than the root, as is the case with the dominant 7 chords. This chord is called D minor 7 or Dm7.

Building four note chords on the C major scale now gives us:

Listen to these chords in order.

We will look more closely at the minor 7 flat 5 chord in a later section.

Since all major scales share the same interval pattern - Tone, Tone, Semitone, Tone, Tone, Tone, Semitone - they also share this pattern of chords. So whatever scale you are looking at, the four note diatonic chord based on the sixth note, for example, will always be a minor 7 chord.

Jazz musicians tend to use these extended chords all the time. Given the chord sequence C Am F G, most jazz people would play Cmaj7 Am7 Fmaj7 G7(or probably something much more complicated).

In part five, we are going to look at how all this theory works when we're not playing in the key of C major.