 Thanks are due to Kenneth Blackwell, Fred Kroon, Jim Robinson and several anonymous referees for their helpful comments on earlier versions of this material and to Allen Hazen for discussions of the second edition of PM and of the iterative conception of sets over many years. Thanks to Andrew Tedder, who checked all the proofs in ∗2 of PM. Thanks to James Toupin, Rodrigo Sabadin Ferreira and Johan Gustafsson for spotting errors in earlier versions of this entry. We are indebted to Axel Boldt for finding a (large) number of errors and also for alerting us to some peculiarities of the PM definitions of sums of classes and of  ordinal similarity in Volume II.  