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  "body": "\n## Summary\n\nAn incidence structure consists of a set of points, a set of lines, and a relation that says which points lie on which lines.\nNothing else is assumed: there are no distances, no angles and no coordinates.\nProjective planes, affine planes, block designs and many other combinatorial objects are incidence structures with extra axioms.\nIt is the basic object of [[incidence geometry|CSXT0BG2CM]].\n\n## Explanations\n\n### L1 human\n\nTake some dots and some curves, and record only which dots sit on which curves.\nThat record is an incidence structure.\nFor example, take the $7$ points and $7$ lines of the Fano plane, where each line has $3$ points and each point is on $3$ lines.\nExchanging the roles of points and lines gives the dual structure, which is again an incidence structure.\n",
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