A short, recurring, instrumental passage first appeared in the early operas. A small section consisting of several musical ideas is stated usually in the beginning as a tutti passage, and this recurs -either in full or in part- at various moments within a movement (most popular in the Baroque era). In a movement, it is expected to appear in full in the tonic in the beginning and then usually in truncated forms in the dominant, relative and subdominant keys and finally in the tonic key in full again. Fundamental to the ‘ritornello structure’ is the alternation of a passage containing the basic thematic substance of the movement (ritornello) with others in which the music is largely or wholly new (episodes). This form was the commonest first movement form in Baroque concertos in the place of a fugal allegro movement in solo and trio sonatas. The tonal structure may be originated as an extension of the first section (AB) of the da capo aria which starts in the tonic with ritornello (R), first episode (A) in the tonic, R in the dominant (V), A’ in the dominant, R in the tonic then part B in relative minor (vi) ending with a Phrygian cadence in the dominant of relative minor (the mediant of the main major key -iii-). (It is also used to describe the tutti passages in a concerto grosso.) The refrain section in a ritornello movement acts as a binding agent. The difference from rondo is that in rondo form, each repeated part is in the tonic key.