 PM makes special use of the distinction between “real”, or free, variables and “apparent”, or bound, variables. Since “\(x\)” is a variable, “\(xRy\)” will be an atomic formula in our extended language, with “\(x\)” and “\(y\)” real variables. When such formulae are combined with the propositional connectives \(\osim\), \(\lor\), etc., the result is a matrix. For example, “\(aRx \ldot {\lor} \ldot xRy\)” would be a matrix.