To understand the nature and value of the evidence thus afforded as to the extent of the mortality, a few words of explanation are necessary. In each diocese there was kept by the Bishop's Registrar a list of all the institutions made to vacant benefices by the Bishop. As a rule, not only was the name of the place and of the out-going and the incoming incumbent, together with the date expressed, but the reason of the vacancy was stated, whether arising from death, exchange, or resignation. These lists, then, for the fatal period, or the autumn of 1348 and the year 1349, afford some means of gauging the extent of the mortality among the clergy. It must, however, be borne in mind that these registers record only the institutions of the actual [p076] incumbents, and take no account of the larger body of curates and chaplains, to say nothing of the monks, canons, and friars of a diocese. It has been calculated by a recent writer that non-beneficed clergy more than equal in numbers the holders of benefices, and that the total number of institutions of a diocese may fairly be doubled in estimating the deaths of the clergy during this epidemic.[130] These Books of Institutions, moreover, by furnishing the dates of the appointments made to various livings, afford a means of determining, at least approximately, the time when the plague was rife in a district, and even, making allowances for any delay in filling up the benefice, in any given place. Besides the special register of each diocese a series of official state documents, called the _Patent Rolls_, contains much evidence of the destructive powers of the disease. On these rolls, amongst every variety of public document, are entered royal grants, licenses, and presentations made by the Sovereign to such vacant ecclesiastical livings as were at the time in the royal gift. These were ordinarily— (1) Benefices of which the King was by right the patron. (2) Those to which he presented, as guardian of the sons of tenants _in capite_ during their minority, and (3) Livings to which bishops and abbots of sees and monasteries, then vacant, ordinarily presented. At this period, 1348-9, moreover, the royal presentations were largely augmented by the patronage attached to the alien religious houses existing in England, the possession of which, "by reason of his war with France," as the official phrase runs, "the King had seized into his own hands."