In addition, about a quarter of net monastic wealth on average consisted of "spiritual" income arising where the religious house held the advowson of a benefice with the legal obligation to maintain the cure of souls in the parish, originally by nominating the rector and taking an annual rental payment. Over the medieval period, monasteries and priories continually sought papal exemptions, so as to appropriate the glebe and tithe income of rectoral benefices in their possession to their own use. However, from the 13th century onwards, English diocesan bishops successfully established the principle that only the glebe and 'greater tithes' of grain, hay and wood could be appropriated by monastic patrons in this manner; the 'lesser tithes' had to remain within the parochial benefice; the incumbent of which thenceforward carried the title of 'vicar'. By 1535, of 8,838 rectories, 3,307 had thus been appropriated with vicarages; but at this late date, a small sub-set of vicarages in monastic ownership were not being served by beneficed clergy at all. In almost all such instances, these were parish churches in the ownership of houses of Augustinian or Premonstratensian canons, orders whose rules required them to provide parochial worship within their conventual churches, for the most part as chapels of ease of a more distant parish church. From the mid-fourteenth century onwards the canons had been able to exploit their hybrid status to justify petitions for papal privileges of appropriation, allowing them to fill vicarages in their possession either from among their own number, or from secular stipendiary priests removable at will; these arrangements corresponded to those for their chapels of ease.