
1) Motif 2) Motive 3) Obligato
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1) Obligato
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https://www.crosswordclues.com/clue/obbligato

- a persistent but subordinate motif
- a part of the score that must be performed without change or omission
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required, indispensable
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http://en.wikipedia.org/wiki/Glossary_of_musical_terminology

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 borro...
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http://en.wikipedia.org/wiki/Obbligato

(Italian: `obligatory`), in music, essential but subordinate instrumental part. For example, in an 18th-century aria with trumpet obbligato, the ... [1 related articles]
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http://www.britannica.com/eb/a-z/o/2

required, indispensable
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http://www.encyclo.co.uk/local/22287

obbligato
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http://www.musictheory.org.uk/res-musical-terms/italian-musical-terms.php

In musical notation, obbligato indicates that an instrument or voice is essential and not to be omitted. However, some composers have used this term with the opposite meaning in mind: that an instrument or voice may be omitted if so desired. Generally, the older the notation, the more likely it is that obbligato retains its original meaning of an '...
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http://www.probertencyclopaedia.com/browse/VO.HTM

required, indispensable
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http://www.translationdirectory.com/glossaries/glossary307.php

[
n] - a persistent but subordinate motif 2. [n] - a part of the score that must be performed without change or omission
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http://www.webdictionary.co.uk/definition.php?query=obbligato
obligato noun a part of the score that must be performed without change or omission
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https://www.encyclo.co.uk/local/20974

An extended solo, often accompanying the vocal part of an aria.
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https://www.encyclo.co.uk/local/21781

indispensable
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https://www.encyclo.co.uk/local/22288

In opera, a solo instrumental part in a vocal number designed not just to accompany but to support the principal vocal part or to throw it into relief. Sometimes as flashy as the vocal part itself.
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https://www.glyndebourne.com/opera-archive/introduction-to-opera/glossary/
No exact match found.