06A05 Total order
06A06 Partial order, general
06A07 Combinatorics of partially ordered sets  
06A11 Algebraic aspects of posets [See also 05E25]
06A12 Semilattices [See also 20M10; for topological semilattices see 22A26]
06A15 Galois correspondences, closure operators
06A99 None of the above, but in this section