In order to return simple facts that match a query, the interpreter must match a query that contains variables with a fact that does not. For example, the query (query (parent abraham ?child)) and the fact (fact (parent abraham barack)) match, if the variable ?child takes the value barack.

In general, a pattern matches some expression (a possibly nested Scheme list) if there is a binding of variable names to values such that substituting those values into the pattern yields the expression.

For example, the expression ((a b) c (a b)) matches the pattern (?x c ?x) with variable ?x bound to value (a b). The same expression matches the pattern ((a ?y) ?z (a b)) with variable ?y bound to b and ?z bound to c.