This chapter explores the relationship between mathematical thinking and properties in Peter Eisenman’s architecture. During his career, Eisenman expressed his design thinking using rigorous sequences of diagrams, depicting the geometric transformation of his ideas into architecture. For Eisenman, mathematics has intrinsic symbolic, semiotic and philosophical potential, allowing him to communicate ideas and record intentions using geometry and numbers. This chapter commences with an overview of Eisenman’s contribution to design and scholarship before tracing his involvement in two movements. First, in the 1960s and 1970s, he proposed a ‘post-functionalistPost-functionalist’ approach, where architecture is freed of the pragmatic shackles—the exigencies of client, program or site—that restrict its potential. Second, in the late 1980s, he was a central figure in the ‘deconstructionist’ movement, which questioned and rejected quotidian architectural values and expressions. The first of these, his earlier formalist period, is the focus of the chapter, as it is in Eisenman’s post-functionalistPost-functionalist works that his mathematical thinking is most visible. This is especially the case in Eisenman’s ‘numbered’ series of houses, the first five of which are amongst the works analysed in Part II of this book: House IHouse I, House II, House IIIHouse III, House IVHouse IV and House VIHouse VI. These five are described and illustrated in this chapter and discussed in the context of ideas embodied in later works in this series (House X, House 11aHouse 11a, House El Even OddHouse El Even Odd andFin d’Ou T Hou S Fin d’Ou T Hou S). Through the analysis of these works, the chapter demonstrates how Eisenman’s translation of design thinking into properties by way of sequential axonometric drawings evolved. This chapter also traces Eisenman’s interest in the formal properties of Le Corbusier and Palladio and his connection to Colin Rowe’s teaching and ideas. It concludes by summarising Eisenman’s applications of mathematics in architectural thinking and the relationship between thinking and properties in his early work.