In classical music obbligato 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; the spelling obligato is not 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.
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 
A later use has the contradictory meaning of optional, indicating that a part was not obligatory. A difficult passage in a concerto might be furnished by the editor with an easier alternative called the obbligato. Or a work may have a part for one or more solo instruments, marked obbligato, that are decorative rather than essential; the piece is complete and can be performed without the added part(s). The traditional term for such a part is ad libitum, or ad lib., or simply "Optional", since ad lib. may have a wide variety of interpretations.
Contemporary usage 
In classical music the term has fallen out of use by modern-day practitioners, as composers, performers and audiences alike have come to see the musical text as paramount in decisions of musical execution. As a result, everything is now seen as 'obbligato' unless explicitly specified otherwise in the score. It is still used to denote an orchestral piece with an instrumental solo part that stands out, but is not as prominent as in a solo concerto, as in Bloch's Concerto Grosso mentioned below. The term is now used mainly to discuss music of the past. One amusing usage, however, is that by Erik Satie in the third movement of "Embryons desséchés" (Desiccated embryos), where the obbligato consists of around twenty F-major chords played at fortissimo (this is satirising Beethoven's symphonic style).
The term is also used with an entirely different meaning, signifying a countermelody.
Explicit instances 
- J.S. Bach used Organ obbligato to show at a glance the importance of the organ part (in for example cantata Wer sich selbst erhöhet, der soll erniedriget werden, BWV 47 and cantata Gott ist mein König, BWV 71).
- Mozart marks "cello obligato" in Don Giovanni in Zerlina's aria Batti, batti, o bel Masetto.
- Beethoven's Duo for viola and cello, WoO 32, is subtitled "mit zwei obligaten Augengläsern" ("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 Schütz's "Benedicam Dominum in omni tempore" in Symphoniae sacrae, i, 1629 for soprano, tenor, bass and continuo with obbligato cornetto, o violino.
- John Philip Sousa's march Stars and Stripes Forever contains a piccolo obbligato in its grandioso.
- Ernest Bloch's 1925 Concerto Grosso No. 1 for string orchestra with piano obbligato is a neoclassical composition with 20th-Century modal harmonies.
- Malcolm Arnold's A Grand, Grand Overture, Op. 57 (1956) is a 20th-century parody of the late 19th century concert overture, and contains obbligato parts for four rifles, three Hoover vacuum cleaners (two uprights in B♭, one horizontal with detachable sucker in C), and an electric floor polisher in E♭
- Benjamin Britten's 1958 Nocturne for tenor, 7 obligato instruments & strings in which the tenor soloist is accompanied by one or more obbligato instruments in each of the eight movements (apart from the first)
Implicit instances 
- Trumpet obbligato in J.S. Bach's cantata Jauchzet Gott in allen Landen, BWV 51
- A horn obbligato during Sifare's aria, Lungi da te, mio bene, in W.A. Mozart's opera Mitridate (1770).
- 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 two arias with obbligato clarinet; bassett clarinet obbligato Parto, ma tu ben mio (sung by Sesto) and basset-horn obbligato Non piu di fiori (sung by Vitellia).
- Piano obbligato in Mozart's concert aria "Ch'io mi scordi di te? … Non temer, amato bene" (K. 505).
- Horn obbligato aria Abscheulicher!/Komm Hoffnung in Beethoven's opera Fidelio.
- An especially ornate violin obbligato appears in the Benedictus of Ludwig van Beethoven's Missa solemnis.
- Corno (horn) obbligato in Gustav Mahler's Symphony No. 5
- 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)
- Bass clarinet obbligato in the third movement of Morton Gould's "Latin American Symphonette"
- Ravel's Daphnis and Chloe Suites may be performed without the chorus parts and are often recorded in this form, an example of the contradictory usage above.
- Piano obbligato in the third movement of Frederik Magle's symphonic suite Cantabile.
- "Obbligato" in The Oxford Dictionary of Music, Oxford University Press: Michael Kennedy (ed.), 1985
- "Obbligato" in Lectionary of Music, Nicolas Slonimsky. McGraw-Hill ISBN 0-07-058222-X
- "Obbligato" in Collins Music Encyclopedia, Westrup & Harrison: Collins, London, 1959