Roman numeral analysis — Wikipedia:

In music, Roman numeral analysis uses Roman numerals to represent chords. The Roman numerals (I, II, III, IV, …) denote scale degrees (first, second, third, fourth, …); used to represent a chord, they denote the root note on which the chord is built. For instance, III denotes the third degree of a scale or the chord built on it. Generally, uppercase Roman numerals (such as I, IV, V) represent major chords while lowercase Roman numerals (such as i, iv, v) represent minor chords (see Major and Minor below for alternative notations); elsewhere, upper-case Roman numerals are used for all chords.[2] In Western classical music in the 2000s, Roman numeral analysis is used by music students and music theorists to analyze the harmony of a song or piece.

A great place to find notation conventions. NNS gives me such grief…