Something like the following graphic is presented at the beginning of nearly every theory book. This represents the character of triads built on each step of the major scale. Note that I, IV and V (the Tonic, Subdominant and Dominant) are major (and upper case), and ii, iii, and vi are minor (lower case). A triad built on the leading tone of the scale is a diminished triad, or the top three notes of the Dominant seventh chord (V7).
These are designated as follows:
I - Tonic
ii - Supertonic
iii - Mediant
IV - Subdominant
V - Dominant
vi - Submediant
vii° - Leading Tone
The following represents the Root Position and First and Second Inversions of the I chord in C Major. These are labeled as: I, I, and I .
Seventh chords have an additional third at the top of the triad:
The root position and the three inversions of the seventh chord are designated as follows: