A "sequence" indicates the repetition of a musical idea in a lower or higher pitch. A sequence is "Tonal" when there are no accidentals added to the repetition of the musical idea in its different pitches. In this way, there can be a difference of a semitone in the intervals between the notes of a sequence.
A sequence is called "real" or "absolute" when the intervals between the different notes are closely guarded, thus having to add accidentals and change the key of the passage. Beethoven uses the "tonal" form of sequence in the opening of his Fifth Symphony.
The sequence is one of the most important characteristics of the Baroque period (Vivaldi, Bach, Handel, and others).
