Question: What was the duty of a nomenclator in ancient times?
Answer: There were a number of unusual professions in ancient times, many of which were carried out by slaves.
In ancient Rome and Greece, long before the BlackBerry and Ipad were introduced some wealthy people had personal assistants who helped them to remember names and personal details of those who would come to greet them.
A nomenclator was basically an ancient living-human computer. It was an important, but difficult job, unless you had excellent memory.
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A nomenclator often accompanied his master to a party and was forced to remember all names and numbers of those his master met. This was especially important when the master had too much too drink. A nomenclator should also be able to tell what the discussion and any socially important information that could be important for a political campaign.
A nomenclator also denotes a person, generally a public official, who announces the names of guests at a party or other social gathering or ceremony.
In more general terms still, it is a person who provides or creates the names for things and this can apply to the application of names in a scientific or any other context but especially in relation to specialist terminologies, glossaries etc.
Source: Vicky Leon: Working IX to V: Orgy Planners, Funeral Clowns, and Other Prized Professions of the Ancient World