Full Idea
Two individuals are 'disjoint' mereologically if and only if they do not overlap, expressed by 'x | y', read as 'x is disjoint from y'. Disjointedness is symmetric.
Clarification
[The vertical stroke actually curves to the right at the lower end]
Gist of Idea
Disjoint: two individuals are disjoint iff they do not overlap, written 'x | y'
Source
Peter Simons (Parts [1987], 1.1.04)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.13