On the Model Theory of Open Incidence Structures: The Rank 2 Case

Taking inspiration from [1, 21, 24], we develop a general framework to deal with the model theory of open incidence structures. In this first paper we focus on the study of systems of points and lines (rank $2$). This has a number of applications, in particular we show that for any of the following classes all the non-degenerate free structures are elementarily equivalent, and their common theory is decidable, strictly stable, and with no prime model: $(k,n)$-Steiner systems (for $2\le k<n$); generalised $n$-gons (for $n\ge 3$); $k$-nets (for $k\ge 3$); affine planes; projective Möbius, Laguerre and Minkowski planes.