Three obligations, i.e. necessitas, owed by AS tenants and landholders: armed service in the *fyrd, the repair of forts and bridge building. These obligations were the only ones ever exempted from any grants of immunities. Sometimes one finds trinoda rather than trimoda; the English phrase 'common burdens' is also used. [< Lat. trimoda = three modes or ways + necessitas = obligation] -
Cf. Thegn
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.