Full Idea
Two parts 'overlap' mereologically if and only if they have a part in common, expressed by 'x o y', read as 'x overlaps y'. Overlapping is reflexive and symmetric but not transitive.
Gist of Idea
Overlap: two parts overlap iff they have a part in common, expressed as 'x o y'
Source
Peter Simons (Parts [1987], 1.1.03)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.11
A Reaction
Simons points out that we are uncomfortable with overlapping (as in overlapping national boundaries), because we seem to like conceptual boundaries. We avoid overlap even in ordering primary colour terms, by having a no-man's-land.