The underlying concept of Chopin's “Nocturnes” probably lay in the romance-like character of adagios and larghettos of piano concertos at the turn of the 18th and 19th centuries as well as the operatic aria of the 'da capo' type. Volume V of the National Edition contains eighteen nocturnes by Chopin. Of the variants in musical notation, those designated 'ossia' were marked in this way by Chopin himself or inscribed in his hand in pupils' copies, while those without this indication result from textual divergences in authentic copies or from difficulties in reading the text in an unambiguous way.
Chopin National Edition
(ed. Jan Ekier)