Knights Service

A form of tenure granted by the King, in consideration for certain services to be performed. After the Norman invasion, England was divided into some sixty-thousand knight's fees, each fee comprising one or more large tracts of land. Each knight was required to perform forty days per year of service in the King's wars, when called upon. This type of land tenure -- also known as chivalry -- was honorable but uncertain in duration, being at the King's pleasure.

Source: Black's Law Dictionary, 4th Edition.