Roman numeral analysis in the context of "Nashville Number System"

In a musical composition, a chord progression or harmonic progression (informally chord changes, used as a plural, or simply changes) is a succession of chords. Chord progressions are the foundation of harmony in Western musical tradition from the common practice era of classical music to the 21st century. Chord progressions are the foundation of popular music styles (e.g., pop music, rock music), traditional music, as well as genres such as blues and jazz. In these genres, chord progressions are the defining feature on which melody and rhythm are built.

In tonal music, chord progressions have the function of either establishing or otherwise contradicting a tonality, the technical name for what is commonly understood as the "key" of a song or piece. Chord progressions, such as the extremely common chord progression I-V-vi-IV, are usually expressed by Roman numerals in classical music theory. In many styles of popular and traditional music, chord progressions are expressed using the name and "quality" of the chords. For example, the previously mentioned chord progression, in the key of E♭ major, would be written as E♭ major–B♭ major–C minor–A♭ major in a fake book or lead sheet. In the first chord, E♭ major, the "E♭" indicates that the chord is built on the root note "E♭" and the word "major" indicates that a major chord is built on this "E♭" note.

In music, the tonic is the first note scale degree () of the diatonic scale (the first note of a scale) and the tonal center or final resolution tone that is commonly used in the final cadence in tonal (musical key-based) classical music, popular music, and traditional music. In the movable do solfège system, the tonic note is sung as do. More generally, the tonic is the note upon which all other notes of a piece are hierarchically referenced. Scales are named after their tonics: for instance, the tonic of the C major scale is the note C.

The triad formed on the tonic note, the tonic chord, is thus the most significant chord in these styles of music. In Roman numeral analysis, the tonic chord is typically symbolized by the Roman numeral "I" if it is major and by "i" if it is minor.

In Western classical music, the common practice period (CPP) was the period of about 250 years during which the tonal system was regarded as the only basis for composition. It began when composers' use of the tonal system had clearly superseded earlier systems, and ended when some composers began using significantly modified versions of the tonal system and developing other systems as well. Most features of common practice (the accepted concepts of composition during this time) persisted from the mid-Baroque period through the Classical and Romantic periods, roughly from 1650 to 1900. There was much stylistic evolution during these centuries, with patterns and conventions flourishing and then declining, such as the sonata form. The most prominent unifying feature throughout the period is a harmonic language to which music theorists can today apply Roman numeral chord analysis; however, the "common" in common practice does not directly refer to any type of harmony, rather it refers to the fact that for over two hundred years only one system was used.

In music, the subdominant is the fourth tonal degree () of the diatonic scale. It is so called because it is the same distance below the tonic as the dominant is above the tonic – in other words, the tonic is the dominant of the subdominant. It also happens to be the note one step below the dominant. In the movable do solfège system, the subdominant note is sung as fa.

The triad built on the subdominant note is called the subdominant chord. In Roman numeral analysis, the subdominant chord is typically symbolized by the Roman numeral "IV" in a major key, indicating that the chord is a major triad. In a minor key, it is symbolized by "iv", indicating that the chord is a minor triad.

Figured bass is musical notation in which numerals and symbols appear above or below (or next to) a bass note. The numerals and symbols (often accidentals) indicate intervals, chords, and non-chord tones that a musician playing piano, harpsichord, organ, or lute (or other instruments capable of playing chords) should play in relation to the bass note. Figured bass is closely associated with basso continuo: a historically improvised accompaniment used in almost all genres of music in the Baroque period of Classical music (c. 1600–1750), though rarely in modern music. Figured bass is also known as thoroughbass.

Other systems for denoting or representing chords include plain staff notation, used in classical music; Roman numerals, commonly used in harmonic analysis; chord letters, sometimes used in modern musicology; the Nashville Number System; and various chord names and symbols used in jazz and popular music (e.g., C Major or simply C; D minor, Dm, or D−; G, etc.).

A secondary chord is an analytical label for a specific harmonic device that is prevalent in the tonal idiom of Western music beginning in the common practice period: the use of diatonic functions for tonicization.

Secondary chords are a type of altered or borrowed chord, chords that are not part of the music piece's key. They are the most common sort of altered chord in tonal music. Secondary chords are referred to by the function they have and the key or chord in which they function. In Roman numeral analysis, they are written with the notation "function/key". Thus, one of the most common secondary chords, the dominant of the dominant, is written "V/V" and read as "five of five" or "the dominant of the dominant". The major or minor triad on any diatonic scale degree may have any secondary function applied to it; secondary functions may even be applied to diminished triads in some special circumstances.