In classical musicobbligato usually describes a musical line that is in some way indispensable in performance. Its opposite is the marking ad libitum. It can also be used, more specifically, to indicate that a passage of music was to be played exactly as written, or only by the specified instrument, without changes or omissions. The word is borrowed from Italian (an adjective meaning fixed; from Latin obligatus p.p. of obligare, to oblige; in English the spelling obligato is also acceptable). The word can stand on its own, in English, as a noun, or appear as a modifier in a noun phrase (e.g. Organ obbligato).
Obbligato includes the idea of independence, as in C.P.E. Bach's 1780 Symphonies "mit zwölf obligaten Stimmen" ("with twelve obbligato parts") by which Bach was referring to the independent woodwind parts he was using for the first time. These parts were also obbligato in the sense of indispensable.
Continuo
In connection with a keyboard part in the baroque period, obbligato has a very specific meaning: it describes a functional change from a basso continuo part (in which the player decided how to fill in the harmonies unobtrusively) to a fully-written part of equal importance to the main melody part.
Contradictory usage
Curiously, a more recent use of the term has the contradictory meaning of optional, indicating that a part was not obligatory1: a difficult passage in a concerto might be furnished by the editor with an easier alternative called the obbligato.
Modern-day usage
The term has fallen out of use by modern-day practitioners, as composers, performers and audiences alike have come to see the musical text to be paramount in decisions of musical execution, and so everything has come to be seen as 'obbligato'. It is now used mainly to discuss music of the past. One amusing usage however, is that by Erik Satie in the third movement of "Dried up Embryos", where the obbligato consists of around twenty F-Major chords played at fff (this is satirising Beethoven's symphonic style)
Examples
Explicit instances
J.S. Bach used Organ obbligato to show at a glance the importance of the organ part (in for example cantata BWV 47 "Wer sich selbst erhöhet, der soll erniedriget werden" and cantata BWV 71 "Gott ist mein König").
Beethoven's Duo for viola and cello, WoO 32, is subtitled "mit zwei obligaten Augenglasern" ("with two [pairs of] obbligato eyeglasses") which seems to refer to the necessity, at the first performance, of spectacles for both Beethoven and his cellist.
Heinrich Schutz's "Benedicam Dominum in omni tempore" in Symphoniae sacrae, i, 1629 for soprano, tenor, bass and continuo with obbligato `cornetto, o violino´.
An especially ornate violin obbligato appears in the Benedictus of Ludwig van Beethoven's Missa solemnis.
Prominent obbligato writing for flute in particular is not unusual in Romantic opera, for example in the cadenza of the traditional version of the Mad Scene in Lucia di Lammermoor (1835)
In W.A. Mozart's opera Mitridate (1770) there is a horn obbligato
In Mozart's Die Entführung aus dem Serail (1782) there are obbligati for flute, oboe, violin and cello.
In Mozart's La Clemenza di Tito (1791) there are arias with obbligato clarinet, and obbligato basset-horn.