This paper focuses on why and how Lacan repeatedly turned to mathematics to speak about his theory of the subject. As is widely known, Lacan discusses mathematical topology and knot theory extensively in his later seminars. This calls into question why his earlier linguistic foundation of psychoanalysis which circles around the signifier and the signified would not succeed to conceptualize that which he now thought of to be of pivotal relevance. This paper argues that it was the need to make something of the Real accessible to experience and theoretization which led Lacan to delve deeper into mathematics. To do so, I will first present some remarks on the disciplinary status of mathematics in Lacan’s structural psychoanalysis. Next, I will revisit the linguistic-based approach Lacan took during his 1950’s seminars. In doing so I will highlight the status of Poe’s story „The Purloined Letter” in terms of signification. Reading „letter” in its two-fold meaning I will then argue that it builds the bridge between linguistics and mathematics for Lacan. The alphabetical letter as used in mathematics can be of use for psychoanalysis as the signifier’s „material” and as such it touches the Real. Also, ciphering and counting are of importance when it comes to answering the question how mathematics provide something psychoanalysis can pick up. This can then lead to discussing the so called borromean knot. By keeping in mind some major thoughts of mathematical topology one can follow Lacan’s aim to formalize the Real in its relation to the Symbolic and the Imaginary. It turns out to be of paramount importance to focus on the knot’s two-dimensional representational form and its three-dimensional, spatial one. Discussing this, two theses can be formed: First, the Real provides the „litoral” for meaning to be formed out of the Symbolic and the Imaginary. Second, the human subject is located in between the second and the third dimension. Reading the dimensional as „dit-mensional” or even as „dit-moins-un“-al can lead to thoughts about the relation between that which is said („le dit“) and that which is shown in its spatial form („dit-mension“) and thus between linguistics and mathematics.