The "da capo aria"

 Composers of vocal music in the Baroque period often used ternary form to structure their music. There are two printed sections; an 'A section' followed by a contrasting 'B section.' On the score the instruction 'da capo al fine' would be written at the end of the 'B section.' This led to pieces being essentially in ternary form - 'ABA.'

It was convention that when one would play the 'A section' for the second time the section was varied from its original, with ornaments and extra notes. This was done simply to ensure interest and contrasts remained in the piece. As a consequence of this, each piece would have an element of individual interpretation.