An obligato is a section of music which is viewed as very important by the composer. When the note “obligato” or “obbligato” appears in a musical composition, it means that the marked section should not be left out or altered, because it is crucial to the piece. The opposite of obligato or “obligatory” would be ad libitum, a marking used to indicate that the relevant section can be altered or left out to suit the needs of the performer or conductor.
The term is derived from the Latin obligare, which means “to oblige.” In Italian, it is written out as obbligato, and this spelling is preferred by many musicians, but “obligato” is also acceptable, especially among English-speaking musicians and composers. As the meaning of this word implies, an obligato section of a composition is not negotiable.
Historically, musicians often adapted, changed, or left out chunks of music, making the obligato notation necessary for composers who wanted to keep certain parts of their works intact. While this might seem strange to modern music fans, since classical music today is often performed as written, this practice was once quite common. Depending on the piece, the term may also be a firm reminder that not only is the music necessary, but that the music should be performed exactly as written, with the instrument it was written for.
Especially in Baroque compositions, it is common to see markings which differentiate between obligato and basso continuo. Typically, a basso continuo is supposed to be an unobtrusive accompaniment to the main melody of the piece, and it is often improvised by the musician in performance. When an obligato notation is used, it means that the musician had a specific vision for the harmonic accompaniment, and he or she would like musicians to stick to it.
Because the tendency to alter classical music as desired has faded, the meaning of the term “obligato” has also shifted in some circles. In some instances, the term is used in a sense opposite of the original meaning, to mark a section of a composition which is superfluous or less important. For example, a composer might offer an obligato alternative to a complicated section in a piece of music for musicians who do not feel up to the original composition. Needless to say, this can be a bit confusing, and it is also irritating to some classical performers and composers.