This is from the Theory section on Seventh Chords.

So we've looked at the E major 7th and C major 7th chords, now let's compare the sound of a major 7th chord to a standard major chord. Here we'll play the C major chord first, closely followed by the C major 7th chord

The first chord is C major. This is a very common chord and is based on the standard major triad of 1, 3, 5 giving us C, E, G.

The chord is strummed from the root note in the following order (lowest to highest string):

C E G C E

Next we remove the first finger and strum again. This actually means we are now including the 7th note, which is B (the open string in this case). So our chord is now 1 3 5 + 7 - C, E, G + B

This C major 7th Chord is strummed in the following order (lowest to highest):

C E G B E

The first thing to notice is the difference in tonality. In other words the two chords have different flavours. Including the 7th note causes this. The seventh note adds a slight amount of dissonance, but it still works with the standard triad very well. The effect given by the major 7th chord is loved in many styles of music and is well worth trying out in your own chord progressions and musical ideas.

The second thing to notice about this major 7th chord is that it is easily played. All we are really doing here is finding the octave C note and removing it to play the note behind. This will always reveal the major 7th note from the underlying major scale. In this case, the B note.

Whenever working out your major 7th chords this is a useful thing to understand. If you know the standard major chord shape - then find the octave and move it down by one fret to reveal the major 7th note. In this case it's particularly easy because the B note is an open string (which is easier to play than a fretted note). However, some chords, such as the previous E major 7th Chord we looked at, will involve re-arranging your fingers a bit more than that.

Bear in mind that some seventh chords will be more tricky to play than standard chord triads. This is mainly down to the fact that we are including an extra note, which often involves using another finger on the fretboard.

Also, there are different TYPES of seventh chord. Here we've looked exclusively at the Major 7th chord, but just as we can alter the notes in the core triad (e.g. major to minor by altering the 3rd note), we also have different 7th notes: the major 7th, minor 7th and diminished 7th notes. These are not to be feared, but treated as new musical dimensions to explore! We'll be looking at those other seventh types next!