How are you able to tell the difference between a third and a seventh major chord?

Seventh chords are the most common extension of the basic 3-note triad you come across.
A seventh chord is built by adding an extra note to a triad which is an interval of a 7th above the root note.
e.g. If you build a triad on C you will use the notes (C-E-G). If you add a another note a 7th above C then you will have C-E-G-B. You have just created a basic seventh chord.

THE 5 TYPES OF SEVENTH CHORDS

There are 5 types of 7th chord you will come across and want to use in your music. Each one gives a characteristic sound which will help you when composing music.