 To remind the reader of the change from talking of propositional functions to relations in extension, two further notational alterations are introduced. Greek letters such as \(\alpha\), \(\beta\), etc. , will be used as variables for ranging over classes as well. The individual variables which are ambiguous with respect to type, “typically ambiguous”, will now also range over classes. A function \(\phi\) of two variables x and y is indicated with the arguments in parentheses after the function variable: \(\phi(x, y)\). A two place relation R holding between x and y is written \(x \relR y\), with the R in “infix” position. The obvious limitation of this notation is that it is not readily extended to three place relations, adding a third variable, say z. We will follow the practice in PM and write \(x \relR y\) for binary relations. PM only requires binary relations for most of the three volumes, although the projected volume IV on geometry would need a notation for “x is between y and z”, as can be seen from Henry Sheffer’s unpublished notes from Russell’s lectures on geometry from 1910 at Cambridge. There he uses the notation \(y\rels{B}(x,y)\) which blends the two styles.