A triad is a three-note chord that is stacked in thirds. The triad’s members, from lowest to highest, are named root, 3rd, and 5th.
Because it is stacked in thirds, a triad presents two sizes of intervals: the 3rd from the root to the 3rd, and the 5th from the root to 5th. The quality of the triad is decided by the qualities of these two intervals. (See the table below for details.) Four kinds of triads are introduced in this chapter: augmented, major, minor, and diminished triads. An augmented triad, for example, includes a major 3rd interval and an augmented 5th (from the root to the 3rd and 5th, respectively). To build a D major triad, it needs a major 3rd interval and a perfect 5th – F-sharp and A, when D is the root.
There are two ways to indicate triads: Roman numerals, such as in classical-tonal music, and chords, as in popular music. The use of Roman numerals underlines the function of tonality based on each triad’s placement on the scale, whereas chords define a specific triad of notes (which makes it easier to read for players). Therefore, C-E-G is I in the key of C major and IV in the key of G, but C-E-G is always indicated as “C” in chord sheets.