We will use Roman numerals to represent chords within a specified key. The Roman numeral “$$\left.\text{I}\right.$$” represents a triad built on $$\hat{1}$$, or the 1st note of the scale. Uppercase Roman numerals represent major triads and lowercase Roman numerals (e.g., “$$\left.\text{i}\right.$$”) represent minor triads. Uppercase Roman numerals with a “$$\left.\text{}{+}\right.$$” are augmented (e.g., “$$\left.\text{III}{+}\right.$$”), and lowercase Roman numerals with a “$$\left.\text{}^{\circ}{}\right.$$” are diminished (e.g., “$$\left.\text{vii}^{\circ}{}\right.$$”). (There is also the Nashville Number System 1 , which uses Arabic numbers for chords instead of Roman numerals.)
 Key Lead-Sheet Root Scale Degree of Root Roman Numeral Notes in Chord $$\text{G}$$ $$\left.\text{B}\text{m}\right.$$ $$\text{B}$$ $$\hat{3}$$ $$\left.\text{iii}\right.$$ $$\text{B}$$–$$\text{D}$$–$$\text{F}^♯$$ $$\text{f}$$ $$\left.\text{G}^{\circ}{}\right.$$ $$\text{G}$$ $$\hat{2}$$ $$\left.\text{ii}^{\circ}{}\right.$$ $$\text{G}$$–$$\text{B}^♭$$–$$\text{D}^♭$$ $$\text{c}$$ $$\left.\text{E}^♭{}\right.$$ $$\text{E}^♭$$ $$\hat{3}$$ $$\left.\text{III}\right.$$ $$\text{E}^♭$$–$$\text{G}$$–$$\text{B}^♭$$
For inversion of Roman numerals, we will use a modified “slash chord” notation until we study Figured Bass. For now, we will write “$$\left.\text{I}\right.$$/3rd” if the 3rd of the $$\left.\text{I}\right.$$ chord is the bass note, for example.
 Key Roman Num. Root Scale Deg. of Root Notes in Chord Bass Note $$\text{g}$$ $$\left.\text{V}\middle/\text{3rd}\right.$$ $$\text{D}$$ $$\hat{5}$$ $$\text{D}$$–$$\text{F}^♯$$–$$\text{A}$$ $$\text{F}^♯$$ $$\text{a}$$ $$\left.\text{iv}\middle/\text{5th}\right.$$ $$\text{D}$$ $$\hat{4}$$ $$\text{D}$$–$$\text{F}$$–$$\text{A}$$ $$\text{A}$$ $$\text{D}$$ $$\left.\text{vii}^{\circ}{}\middle/\text{3rd}\right.$$ $$\text{C}^♯$$ $$\hat{7}$$ $$\text{C}^♯$$–$$\text{E}$$–$$\text{G}$$ $$\text{E}$$
en.wikipedia.org/wiki/Nashville_number_system