- "Expression or Expression" -- see Expression -- the class of all expressions
- "Expression or Holder" -- see Expression -- the class of all expressions
- "Expression or Thing" -- see Expression -- the class of all expressions
- "Holder or Expression" -- see Expression -- the class of all expressions
- "Holder or Holder" -- see Expression -- the class of all expressions
- "Thing or Expression" -- see Expression -- the class of all expressions

This operator may be used as a binary operator in an expression like x or y. The user may install binary methods for handling such expressions with code such as

X or Y := (x,y) -> ...

where X is the class of x and Y is the class of y.