 PM also has Greek letters for classes: \(\alpha, \beta, \gamma\), etc. These will appear as bound (real) variables, apparent (free) variables and in abstracts for propositional functions true of classes, as in \(\phi \hat{\alpha}\). Only definitions of the bound Greek variables appear in the body of the text, the others are informally defined in the Introduction:   \[ \tag*{∗20·07}   (\alpha) \sdot f \alpha \ldot {=} \ldot  (\phi) \sdot  f \{ \hat{z}(\phi\bang z)\} \quad\Df \]   or, in modern notation,  \[ \forall \alpha\, f\alpha =_\mathit{df} \forall \phi f\{z\mid\phi z\} \]   where \(\phi\) is a predicative function. 