Two notes forming a harmonic interval are the simplest example of different notes performed together. The next simplest combination is the triad, which – as its name implies – consists of three notes. These three are the note on which the triad is based (the root) plus the 3rd and the 5th above it-
Triads can be built on each degree of the major and minor scales. They take their names from the degrees of the scale on which they are based: the triad on the tonic (key-note) is the “tonic triad”; the triad on ht dominant is the “dominant triad”; and so on. As a short hand device they are also referred to by roman numerals: I, II, III, IV, V, VI, VII for tonic, supertonic,mediant, etc.
The triads on I, IV, and V are known as the “primary” triads while the others are secondary triads.

Triads are classified as major, minor, augmented or diminished. A major triad consists of a major 3rd and a perfect 5th, a diminished triad consists of a minor 3rd and diminished 5th; and an augmented triad consists of a major 3rd and an augmented 5th.

For example, on C major, we can see that in a major key, the triads on I, Vand V are major; those on II, III and VI are minor; while the triad on VII is diminished.
In a minor key the tonic triad (I) is always minor, but triads on all other degrees of the scale are variable because of the alternative forms of the 6th and 7th degrees derived from the harmonic and melodic scales. Thus the key of C minor produces the following triads –

All the triads so far have been in “root position”, because the root is the bottom note in each case. But the notes of a triad can be rearranged so that either the 3rd or the 5th is at the bottom –
When the 3rd is at the bottom, the triad is in root position; when the 5th is at the bottom it is in the “second inversion”.
When roman numerals are used to denote triads, the small letters “a”, “b” and “c” maybe added to indicate “root position, ‘first inversion’ and ‘second inversion’ respectively. Thus ‘Vb’, for example, means the first inversion of the dominant triad, and ‘IVc’ means the second inversion the second inversion of the subdominant triad.
If a triad is arranged with all its notes as close to each other as possible, it is said to be in ‘close position’. An open position triad is still described as being in root position, or in first or second inversion, according to which of its notes is at the bottom