Chords are numbered in Roman numerals. For example, in the scale of C major, the chords are numbered like this: An added 6th is a major triad with the addition of a 6th note above the root.
All the chords built on other notes in the scale of C are minor, except chord VII - the notes B – D – F, which is diminished. When a seventh is added to a chord this is known as a seventh chord.