part_of part of is part of part of contained in RO:0001018 contained_in Containment is location not involving parthood, and arises only where some immaterial continuant is involved. Containment obtains in each case between material and immaterial continuants, for instance: lung contained_in thoracic cavity; bladder contained_in pelvic cavity. Hence containment is not a transitive relation. If c part_of c1 at t then we have also, by our definition and by the axioms of mereology applied to spatial regions, c located_in c1 at t. Thus, many examples of instance-level location relations for continuants are in fact cases of instance-level parthood. For material continuants location and parthood coincide. Containment is location not involving parthood, and arises only where some immaterial continuant is involved. To understand this relation, we first define overlap for continuants as follows: c1 overlap c2 at t =def for some c, c part_of c1 at t and c part_of c2 at t. The containment relation on the instance level can then be defined (see definition): Intended meaning: domain: material entity range: spatial region or site (immaterial continuant) contained in contained_in contains located in located in independent continuant independent continuant material entity material entity