[...] we identify a period concept with the term plus a reference to the first known publication that actually defined it, just as it is good practice in biology (Berendsohn 1995). We propose to create unique preferred terms as in biology (e.g.: ‘Fringilla coelebs Linnaeus, 1758’). If details of the original definition have undergone some evolution, we would refer to the most recent interpretation of it. This case is distinct from an actual redefinition, which would give raise to a new concept. In the case of diverging interpretations, we also propose to create two distinct definiti on documents. Any number of synonyms in any natural language may be added. Synonyms need not be unique. They guid e the user to the concept and preferred term. A URI (“universal resource identifier”) may serve as language-neutral, machine-processable identifier