Even when the printers had learnt how to print two folio pages at the same time the presswork remained very laborious. The earliest presses were worked with only a single screw, and when the pressman had pulled the lever one way to bring the platen down on the type, he had to push the lever back again in order to raise the platen and release the paper. Thus in order to print a large book quickly four or six sets of pressmen had to work on it at once, each at a different press. To avoid mistakes, therefore, the practice was to allot one section of the book to each press. Thus if a book were calculated to run to 288 leaves, six presses might begin simultaneously at leaves 1, 49, 97, 145, 193, and 241. What more often happened was that either to follow the natural sections of the book, or because some of the printers were engaged on other tasks and not ready to begin at once, the sections were of much less regular lengths, and we can sometimes prove that the first press was far advanced in its section before the fifth and sixth had begun. Now in all these cases, unless they were reprinting an earlier book, page for page, it is obvious that some nice calculations would be needed to make each section end with the end of a quire so as to be able to join on with the beginning of the quire containing the next section without any gap or crowding. Hence the striking irregularities in the make-up of many early books. Instead of a book being printed in a succession of quinternions or a succession of quaternions we have many a make-up which can only be expressed by a cruelly mathematical formula, such as this, which represents the quiring of the Forty-two Line Bible. a-i^10; k^10+1 lm^10 n^6+1; o-z^10 [inverted 2]^10 [@]^10+1; A-F^10 G^4: aa-nn^10; oo pp^10 qq^10+1; rr-zz AA-CC^10; DD^12 EE^10+1; FF GG^10 HH^4+1 II^10. In this the index-letter shows the number of leaves in the quire, a-i^10 being a short way of stating that each of the nine quires a b c d e f g h i has ten leaves in it. In the tenth quire (k) there is an extra leaf, and again in the thirteenth (n) the printer found that he had too much copy for six leaves and not enough for eight, and was therefore obliged to put in an odd one, because another press had already printed off the beginning of the next quire (o). Not infrequently it would happen that the odd amount of copy for a section was very difficult to fit exactly into a leaf even when the printer had compressed it by using as many contractions as possible, or eked it out by using no contractions at all. This accounts for the occurrence of a blank space, large or small, at the end of some sections without any break in the text, as the printer was sometimes careful to explain by the printed notice "Hic nihil deficit," or as in our page from Ulrich Zell, "Vacat."