Concise AACR2: 40. Titles of nobility

In the case of the name of a nobleman or noblewoman not entered under title (see rule 35), add the title of nobility in the person’s language if the title or part of the title commonly appears with the name in works by the person or in reference sources.4 In case of doubt, omit the title.
Disregard reference sources dealing only with the nobility and gentry.

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