In music, a tone row or note row (German: Reihe or Tonreihe), also series or set, is a non-repetitive ordering of a set of pitch-classes, typically of the twelve notes in musical set theory of the chromatic scale, though both larger and smaller sets are sometimes found.

Tone rows are the basis of Arnold Schoenberg's twelve-tone technique and most types of serial music. Tone rows were widely used in 20th-century contemporary music, like Dmitri Shostakovich's use of twelve-tone rows, "without dodecaphonic transformations."

A tone row has been identified in the A-minor prelude, BWV 889, from Book II of J.S. Bach's The Well-Tempered Clavier (1742). It is also found in works such as Mozart's String Quartet in C, K. 157 (1772), String Quartet in E♭, K. 428, String Quintet in G minor, K. 516 (1790), and the Symphony No. 40, K. 550 (1788). A passage from Symphony No. 40 is shown below in which every tone in the chromatic scale is played except for G (the tonic):

Beethoven also used the technique but, on the whole, "Mozart seems to have employed serial technique far more often than Beethoven". Franz Liszt used a twelve-tone row in the opening of his Faust Symphony. Hans Keller claims that Schoenberg was aware of this serial practice in the classical period and that "Schoenberg repressed his knowledge of classical serialism because it would have injured his narcissism."

Tone rows are designated by letters and subscript numbers (e.g.: RI₁₁, which may also appear as RI11 or RI–11). The numbers indicate the initial (P or I) or final (R or RI) pitch-class number of the given row form, most often with c = 0.

A twelve-tone composition will take one or more tone rows, called the "prime form", as its basis plus their transformations (inversion, retrograde, retrograde inversion, as well as transposition). These forms may be used to construct a melody in a straightforward manner as in the fifth movement from Schoenberg's Piano Suite, Op. 25, where P-0 is used to construct the opening melody and later varied through transposition, as P-6, and also in articulation and dynamics. It is then varied again through inversion, untransposed, taking form I-0. However, rows may be combined to produce melodies or harmonies in more complicated ways, such as taking successive or multiple pitches of a melody from two different row forms.

Initially, Schoenberg required the avoidance of suggestions of tonality—such as the use of consecutive imperfect consonances (thirds or sixths)—when constructing tone rows, reserving such use for the time when the dissonance is completely emancipated. Alban Berg, however, sometimes incorporated tonal elements into his twelve-tone works. The main tone row of his Violin Concerto hints at this tonality:

This tone row consists of alternating minor and major triads starting on the open strings of the violin, followed by a portion of an ascending whole-tone scale. This whole-tone scale reappears in the second movement when the chorale "Es ist genug" ("It is enough") from J.S. Bach's cantata O Ewigkeit, du Donnerwort, BWV 60 is quoted literally in the woodwinds (mostly clarinet).