18Bxx
18A | 
18 | 
18C 18Bxx | 18A | 18 | 18C |
| 18B05 | Category of sets, characterizations [See also 03-XX] |
| 18B10 | Category of relations, additive relations |
| 18B15 | Embedding theorems, universal categories [See also 18E20] |
| 18B20 | Categories of machines, automata, operative categories [See also 03D05, 68Qxx] |
| 18B25 | Topoi [See also 03G30] |
| 18B30 | Categories of topological spaces and continuous mappings [See also 54-XX] |
| 18B35 | Preorders, orders and lattices (viewed as categories) [See also 06-XX] |
| 18B40 | Groupoids, semigroupoids, semigroups, groups (viewed as categories) [See also 20Axx, 20L05, 20Mxx] |
| 18B99 | None of the above, but in this section |