Chivalry became established as part of the national constitution when William the Conqueror divided the country into about sixty thousand knights' fees, with the tenure of military service. The clergy, as well as the laity, were compelled to furnish armed knights, on horseback, as the price of their possessions, when the king went abroad against his enemies; and, consequently, knights became attached to every ecclesiastical foundation. These servants of the church were generally younger members of baronial families; and as there was constant occasion for them, chivalry became a military profession. In England, as in every country, the feudal array was found insufficient for foreign wars, and wide-spread domestic rebellions; for few contests could be finished in forty days,--and that was the brief space which, in the earliest simplicity of feudal times, had been fixed for the duration of military service. As petty states swelled into kingdoms, and their public operations became extensive, many a martial enterprise was broken up before achievement, because the time of service had expired. So frequent were the calls on the holders of knights' fees, that they were glad to compromise for attendance by pecuniary penalties. The sovereigns were exorbitant in their exactions, in order to be able to pay the stipendiary substitutes; but one of the most important provisions of Magna Charta gave to parliament alone the power of imposing this escuage or military tax.[383] When the custom of escuage arose is a matter which no antiquarian researches have settled. The clause in Magna Charta shows not only its existence, but its being used as an instrument of tyranny; and under this aspect of chivalric history, the reign of John is important. Most of these stipendiary subsidiaries were knights, with their equipments of men-at-arms and archers; and the sovereign was accustomed to contract with his barons for their attendance upon him in his foreign expeditions. Chivalry and feudal tenure were, therefore, no longer convertible terms; yet the spirit of knighthood long survived the decay of the forms of feudal obligation; for the practice of escuage was fully established in the days of Edward III.; and that was the brightest era of English chivalry. [Sidenote: Knighthood a compulsory honor.]