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  "body": "\n## Summary\n\nA hyperplane arrangement is a finite set of hyperplanes in a vector space or affine space.\nOver the real numbers the hyperplanes cut the space into convex pieces called regions.\nOne studies how many regions there are and how they fit together.\nThe combinatorics of an arrangement is captured by its intersection poset, the [[partially ordered|0QV2JS9RJ3]] set of all nonempty intersections of its hyperplanes.\nArrangements connect polytopes, [[matroids|PX31ZNYS3Z]], root systems and the topology of complements of hypersurfaces.\n\n## Explanations\n\n### L1 human\n\nDraw a few straight lines on a sheet of paper.\nThey cut the sheet into pieces, and the question is how many pieces there are.\nThree lines in general position give $7$ pieces, while three parallel lines give only $4$.\nA hyperplane arrangement is the same picture in any dimension, with flat walls instead of lines.\n",
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