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b. Add the opus number, if there is one. This will usually be the unique identifying number for the work. The opus number is separated from the Cutter number and title letters by a space, with no period. Do not add a space between "op." and the number itself. For major composers whose works are routinely referred to by thematic catalog numbers, the thematic catalog number is to be preferred.
Mus.B1223So op.5uniform 5 uniform title = Sonatas, keyboard instrument, op.5
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