1 In mus. construction, the more or less exact repetition of a passage at a higher or lower level of pitch. If the repetition is of only the melody it is called a melodic sequence; if it is of a series of chords it is a harmonic sequence. If the intervals between the notes of the melody are to some extent altered (a major interval becoming a minor one and so forth, as is practically inevitable if the key is unchanged) it is called a tonal sequence; if there is no variation in the intervals (usually achieved by altering not merely the pitch of the notes but also the key) it is called a real sequence. If there are several repetitions, some of them tonal and some real, the result is a mixed sequence. A harmonic real sequence is sometimes called rosalia (some authorities, however, require as an additional qualification for this description a rise of one degree of the scale at each repetition).
2 In ecclesiastical use the term sequence is applied to a type of hymn which began as one of the many forms of interpolation in the original liturgy of the Western Christian Church. As the traditional plainsong did not provide for such interpolations, special melodies were composed. In the Church's service sequences follow (whence the name) the gradual and alleluia. The earliest sequences were in prose, not, as later, in rhymed verse, and the term ‘prose’ is still sometimes used instead of ‘sequence’. The following are examples of the sequence: Dies Irae (now a part of the Requiem), Veni Sancte Spiritus, Lauda Sion, and Stabat Mater dolorosa.