x_{1} is an (arbitrary) x_{2}-set (li) of superset x_{3}; x_{1} is subset/subgroup/subcategory/subclass/vel sim. of x_{3} with cardinality/size x_{2}.

x_{1} is empty if(f) x_{2 }= 0, which is possible; x_{1} may or may not be a proper substructure (praperi) of x_{3}. x_{2} is a nonnegative cardinal. The two distinguishing features of x_{1} are its size (x_{2}) and the object/structure (x_{3}) to which it belongs/which contains it. Any x_{2} elements of x_{3} can belong to x_{1} as long as the total count is correct; no particular collection is necessarily included. It is bad form for x_{2} to strictly exceed the size/cardinality of x_{3} and, necessarily, no such object/structure can exist. See also: klesi, praperi, cletu.

- praperi
- x
_{1}is a strict/proper sub-x_{2}[structure] in/of x_{3}; x_{2}is a structure and x_{1}and x_{3}are both examples of that structure x_{2}such that x_{1}is entirely contained within x_{3}(where containment is defined according to the standard/characteristics/definition of x_{2}; but in any case, no member/part/element that belongs to x_{1}does not also belong to x_{3}), but there is some member/part/element of x_{3}that does not belong to x_{1}in the same way.