A minor triad has a minor third and a perfect fifth, i.e., it is a minor third with a major third on top of it. Ex. C—E♭—G.
A diminished triad has a minor third and a diminished fifth, i.e., it is a minor third with another minor third on top of it. Ex. C—E♭—G♭.
An augmented triad has a major third and an augmented fifth, i.e., it is a major third with another major third on top of it. Ex. C—E—G♯.
198. A triad may be built on any scale-tone, but those on I, IV, and V, are used so much oftener than the others that they are often called the common chords. In referring to triads the Roman numerals are used to show on what scale-tone the triad is based, the size of the numeral (with other signs) indicating the kind of triad found on each tone of the scale. Thus e.g., the large I shows that the triad on the first tone (in major) is a major triad, the small ii shows that the triad on the second tone is minor, etc. The following figure will make this clear.
The triads in the minor scale are as follows:
199. A triad is said to be in fundamental position when its root is the lowest tone. It is said to be in the first inversion when the third is the lowest tone, and in the second inversion when the fifth is the lowest tone. Thus e.g., in Fig. 66 the same chord (C—E—G) is arranged in three different positions, at (a) in fundamental position, at (b) in the first inversion, and at (c) in the second inversion.