Trinoda necessitas

Trinoda necessitas ("three-knotted obligation" in Latin) is a term used to refer to a "threefold tax" in Anglo-Saxon times. Subjects of an Anglo-Saxon king were required to yield three services: bridge-bote (repairing bridges and roads), burgh-bote (building and maintaining fortifications), and fyrd-bote (serving in the militia, known as the fyrd). Rulers very rarely exempted subjects from the trinoda necessitas, because these services were the lifeblood of an Anglo-Saxon kingdom. After the Norman Conquest, exemptions from the trinoda necessitas became more common.