In music theory, a triad is a set of three notes that form a chord. The bottom note of the triad is its root; the second note is a third above the root, and the top note is a fifth above the root. Like thirds, triads can be major or minor, and like fifths, they may be augmented or diminished. Triads are among the principal building blocks of chord structures, and music students learn to identify them in order to analyze the progressions in songs.
Identify the root of the triad, and identify the sharps and flats in that note's key signature. For example, in the triad C-E-G, the root is C, and there are no sharps or flats in the key of C.
Look at the middle note of the triad. Determine whether it is a minor third or a major third above the root by seeing whether it fits in the root's key signature or by counting half-steps. Four half-steps are a major third; three half-steps are a minor third.
Look at the top note of the chord, called the fifth, and determine its interval from the third. Again, count half-steps or consider the key signature, this time using the third as the key center. For example, in a C-E-G triad, recognize that the key of E major has a G#; since the triad instead has a G natural, identify the interval from E to G as a minor third, not a major third.
Put the two intervals together to identify the triad. A major third on the bottom and a minor third on top is a major triad; a minor third on the bottom and a major third on top is a minor triad. Two major thirds together are an augmented triad; two minor thirds together are a diminished triad.
Memorize the key signatures to be able to identify intervals quickly. Memorize the rules listed in Step 4 to identify triads quickly.
Don't get confused by the fact that the middle note of the triad is called the third and the top note is called the fifth. These names refer to their scale degrees from the root, not their order in the triad.