Some pages of the First Folio – 134 out of the total of 900 – were proofread and corrected while the job of printing the book was ongoing. As a result, the Folio differs from modern books in that individual copies vary considerably in their typographical errors. There were about 500 corrections made to the Folio in this way. These corrections by the typesetters, however, consisted only of simple typos, clear mistakes in their own work; the evidence suggests that they almost never referred back to their manuscript sources, let alone tried to resolve any problems in those sources. The well-known cruxes in the First Folio texts were beyond the typesetters' capacity to correct.