Obbligato
Indispensable, soloistic.
What it means
Obbligato means “obligatory, indispensable” and designates a part that may not be omitted. It is a fixed component of the piece and usually carries its own melodic material, often of a soloistic character.
Its counterpart is ad libitum: a part so designated is dispensable, and the work remains complete without it. The distinction played a particular role in the music of the seventeenth and eighteenth centuries, when scorings were often adapted to the available musicians.
Bear in mind that the word has undergone a shift in meaning over time. In more recent usage, especially in English, it is sometimes used for an added, accompanying part — that is, almost in the opposite sense. In the notated text of older music, the original meaning holds.
In the notation
The word stands in the scoring indication or above the staff of the part concerned. In arias it often designates a solo instrument that stands as an equal partner opposite the singing voice. The formulation “cembalo obbligato,” for instance, means that the keyboard instrument has a written-out part instead of merely realizing the figured bass.
Musical use
Obbligato parts shape the chamber and vocal music of the Baroque. In arias a solo instrument appears as a partner of the singing voice; in sonatas the keyboard instrument moves from accompanist to an equal co-player. This development contributes substantially to the emergence, in the eighteenth century, of chamber music with independent parts.
In the Baroque, obbligato designated a part that may not be omitted, for instance a solo flute. Today it also means an independent accompanying part that concerts with the soloist, for instance an obbligato violin in an aria.
Performance practice
Treat an obbligato part as an equal, not as accompaniment: it carries its own material and may come to the fore. Coordinate with the main voice on phrasing and dynamics, so that a dialogue results. In arias, take care that the singing voice remains intelligible where it carries the text. And check the meaning of the word in the context of the edition, since it has changed.