 Then, for example, the pointclass formula_1 of all open sets means the collection of all open subsets of one of these product spaces.This approach prevents formula_1 from being a proper class, while avoiding excessive specificity as to the particular Polish spaces being considered (given that the focus is on the fact that formula_1 is the collection of open sets, not on the spaces themselves).The pointclasses in the Borel hierarchy, and in the more complex projective hierarchy, are represented by sub- and super-scripted Greek letters in boldface fonts; for example, formula_4 is the pointclass of all closed sets, formula_5 is the pointclass of all Fσ sets, formula_6 is the collection of all sets that are simultaneously Fσ and Gδ, and formula_7 is the pointclass of all analytic sets.