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Diatonic harmony refers to chords built exclusively from the notes of a given scale. In a major key, each scale degree generates a triad: I and IV and V are major, ii and iii and vi are minor, and vii° is diminished. Roman numerals label these chords (uppercase for major, lowercase for minor), making the analysis transposable to any key. The I, IV, and V chords are the 'primary' chords that form the harmonic backbone of most tonal music; ii, iii, and vi are 'secondary' chords with related functions.
Build all seven diatonic triads in C major and label them with Roman numerals. Then transpose the analysis to G major and F major to see that the Roman numeral pattern stays the same. Analyze a simple folk song using Roman numerals.
When you learned to build triads, you discovered that the quality of a chord (major, minor, diminished) depends on which thirds are stacked. Diatonic harmony takes this one step further: if you build a triad on every note of a major scale using only the notes already in that scale, a predictable pattern of chord qualities emerges. This pattern is the same in every major key, which is why Roman numeral analysis is so useful — you learn it once and it works everywhere.
In any major key, the seven diatonic triads follow this quality pattern: I major, ii minor, iii minor, IV major, V major, vi minor, vii° diminished. The uppercase Roman numerals (I, IV, V) signal major chords; the lowercase (ii, iii, vi) signal minor chords; and the degree symbol (vii°) signals diminished. The reason this pattern is fixed is that major scale intervals are fixed — the whole- and half-step structure of the scale always places the same interval relationships between scale degrees.
The most important chords to internalize first are the primary chords: I (tonic), IV (subdominant), and V (dominant). These three chords between them can harmonize almost any melody in a major key, and they form the backbone of folk songs, hymns, blues, and pop music. The secondary chords — ii, iii, vi — have related functions: ii tends to move toward V (a common pre-dominant), vi often substitutes for I (sharing two of its three pitches), and iii is rarer and sometimes functions like a tonic substitute.
A common stumbling block is the vii° chord. Students sometimes expect it to be minor (since it is built on the leading tone, a note that is "almost" the tonic), but the scale forces both intervals to be minor thirds, yielding a diminished triad. The tritone between the root and fifth of vii° gives it tremendous instability and a powerful drive to resolve to I — a useful property that composers exploit constantly.
Practicing Roman numeral analysis means learning to read chords relative to a key rather than by their absolute names. When you see a chord labeled IV, you immediately know it is major and built on the fourth scale degree of the current key — regardless of what key that is. This flexibility is what makes Roman numeral notation the standard tool for harmonic analysis across all of tonal music.
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