7. The gatherings

The Gutenberg Bible is described as a 'folio'; this refers to how the paper has been folded.

A book in folio is made up by sheets of paper or vellum folded once in the middle, making up two leaves (or four pages). Several folded sheets are gathered together in quires or gatherings, one being inserted into the other. Each gathering should therefore have an even number of leaves, but in practice it is from time to time necessary to cut out a leaf or to insert an additional one.

In the Gutenberg Bible the gatherings are made up in the following way:

Volume I (324 leaves):
Gatherings 1- 9 have 10 leaves each
Gathering 10 has 11 leaves, an additional leaf being inserted after leaf 8
Gatherings 11-12 have 10 leaves each
Gathering 13 has 7 leaves, an additional leaf being inserted after leaf 4
Gatherings 14-24 have 10 leaves each
Gathering 25 has 11 leaves, an additional leaf being inserted after leaf 7
Gathering 26 has 11 leaves, an additional leaf being inserted after leaf 9
Gatherings 27-32 have 10 leaves each
Gathering 33 has 4 leaves

Volume II (319 leaves):
Gatherings 1-15 have 10 leaves each
Gathering 16 has 11 leaves, an additional leaf being inserted after leaf 10
Gatherings 17-26 have 10 leaves each
Gathering 27 has 12 leaves
Gathering 28 has 11 leaves, an additional leaf being inserted after leaf 7
Gatherings 29-30 have 10 leaves each
Gathering 31 has 5 leaves, an additional leaf being inserted after leaf 3
Gathering 32 has ten 10 leaves

For a good introduction to the folding of sheets into gatherings, see Philip Gaskel, A New Introduction to Bibliography (Oxford, 1972), with several later editions.