51-00 General reference works (handbooks, dictionaries, bibliographies, etc.)
51-01 Instructional exposition (textbooks, tutorial papers, etc.)
51-02 Research exposition (monographs, survey articles)
51-03 Historical (must also be assigned at least one classification number from Section 01)
51-04 Explicit machine computation and programs (not the theory of computation or programming)
51-06 Proceedings, conferences, collections, etc.
51Axx Linear incidence geometry
51Bxx Nonlinear incidence geometry
51C05 Ring geometry (Hjelmslev, Barbilian, etc.)
51Dxx Geometric closure systems
51Exx Finite geometry and special incidence structures
51Fxx Metric geometry
51G05 Ordered geometries (ordered incidence structures, etc.)
51Hxx Topological geometry
51Jxx Incidence groups
51Kxx Distance geometry
51Lxx Geometric order structures [See also 53C75]
51Mxx Real and complex geometry
51Nxx Analytic and descriptive geometry
51P05 Geometry and physics (should also be assigned at least one other classification number from Sections 70--86)