An earlier article on Saenredam's construction drawings (Note, 1 ) left open the question of how he obtained his knowledge of perspective. His teacher Frans de Grebber (Note 2) will no doubt have taught him the rudiments thereof, but the minimal nature of the knowledge thus gained clearly emerges from a study of what is probably his first drawing of a church interior (Fig.1, Note 3) . This drawing of St. Bavo's, Haarlem, which is dated 1627, belongs to a series he made for the third edition of Samuel Ampzing's Beschryvinge ende lof der stad Haarlem.., which was published in 1628 (Note 4). The drawing was made on the spot and served as the direct model for Jan van de Velde's engraving (Note 5), thus there was no intervening construction drawing here. Saenredam did, however, draw some guidelines- orthogonals and vertical axes - with the aid of a ruler. At first sight he appears to have kept fairly carefully to the rules of central perspective, but closer inspection shows that he failed to solve the problem of the rendering of a very large angle of vision (Note 6) . The making of a genuine construction drawing demands a much greater knowledge of perspective and as Saenredam's first construction drawing already dates from 1628 (Note 9), he must have begun his studies of the subject in that year or in 1627. In 1935 Swillens suggested three people who could have helped him: Jacob van Campen, Salomon de Bray and Bartholomeus van Bassen (Note 10). Van Bassen, who in 1639 became city architect of The Hague, where he had worked since 1622, almost certainly commissioned Saenredam to make the drawing of the Koningshuis in Rhenen in 1644 (Note 12), but no other contacts between the two are known. Similarly, although Saenredam made some copies of drawings by Salomon de Bray at his request in 1632 (Note 13) and the two men both served on the board of the Haarlem Guild of St. Luke in 1633-4 and 1640-1 (Note 14), no other evidence of a relation ship exists. Saenredam did, however, know Jacob van Campen from the period 1612 -14, when they were both pupils of De Grebber (Note 15) and he also carried out various commissions for Van Campen later (Note 16), while in 1627 - 8 Van Campen was likewise working on Ampzing's book and in 1628 he drew a portrait of Saenredam (Note 17). However, although Van Campen zvas probably a gifted perspectivst (Note 18), there still remains another candidate with a stronger claim to have been Saenredam's teacher, namely the surveyor Pieter Wils, who was also a mathematician, astronomer and fortification engineer (Note 19). He drew the ground plan of St. Bavo's for Ampzing's book and, much more significantly, he also compiled a list of measurements of the church for the benefit of those wishing to make perspective drawings of it, which was included in the appendix (Note 20). It must be remembered that making drawings of existing churches in perspective with the aid of measurements was an entirely new idea in 1628, so that it seems more than likely that Saenredam will have consulted Wils about his difficulties in making his drawing and that the list was one of the outcomes of this. This supposition is much strengthened now that it has appeared that the three sketchy measurements of fragments, of the interior of St. Bavo's traditionally atrributed to Saenredam were made in preparation for Wils' list (Note 21). Saenredam may also have got his knowledge in part from books on the subject, although there is no published treatise in which all the methods used by him are described (Note 22) and in 1627- 8 there was not even a text describing how to translate the distance between the eye and a given point on the object into the distance on the panel (Note 23). Moreover, 16th- and 17th-century treatises on perspective were in general scarcely suited to self study, being often prolix or even incorrect in their examples (Note 24) or peppered with misprints (Note 25), while the didactic abilities of their authors sometimes left a lot to be desired (Note 26). We now know what books Saenredam possessed, thanks to the recent discovery of a catalogue of the sale of them in Haarlem on 20 April 1667 (Fig. 2, Note 28). Fifteen mumbers in the catalogue relate to books on mathematics, perspective and architecture and a list of these is given here. It is, however, striking how few books on perspective Saenredam possessed. The five works by Steven that he owned (Note 33), for example, did not include the one on perspective, albeit it is most interesting to note that his copies of the first two volumes of Wisconstighe Ghedachtnissen (folio no. 56) were annotated by Pieter Wils, to whom they had originally belonged. If one takes away the books not directly concerned with perspective and those published after 1627- 8, that leaves only Serlio and Dürer (folios no. 19 and 45, Notes 35 and 38) and Saenredam cannot have learned much from either of these that will have been of any practical use to him. Thus it must have been Pieter Wils in the main who helped him to develop usable construction techniques.