What is another word for "key"?

Roget's Thesaurus (1911) lists these synonyms for "key": accession, accoucheur, accoucheuse, agency, agent, agentship, aid, along, and, appointment, assignment, assistance, attorney, authority, bird, brevet, captive, capture, charge, charter, clerkship, close, commission, conducive, consignee, consignment, coronation, crook, custody, delegate, delegation, deputation, deputy, designation, dint, diploma, doing, durance, embassy, enthronement.