 The object names and variables denote, or take values in, a domain of objects and the \(n\)-place relation names and variables denote, or take values in, a domain of \(n\)-place relations. Objects and relations are to be regarded as mutually exclusive domains: no object is a relation and no relation is an object. When giving examples of \(n\)-place relation names or variables for \(n\geq 2\), we often write \(R, S,\ldots\) instead of writing \(P^2,Q^2,\ldots\). 