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FN. 19: “[...] Thoenes suggests that the first version (probably 1519) perhaps written with Baldassare Castiglione, reflected the collaboration of Raphael's co-architect at St. Peter, Antonio da Sangallo the Younger, who was the first architect to consistently employ orthogonal projection. Antonio was the only early-sixteenth-century architect to have been trained as such, rather than as a figural artist. [...]”

FN. 20: “Thoenes (1993), loc. cit., assumes, that Raphael meant an orthogonal section as in Fig. 18, even if he did not say as much. If Sangallo the Younger did contribute to the writing of this passage, as Thoenes believes, such a conclusion would be likely, since he was the principal developer of the method in his generation. [...]”