As with our modern books, medieval manuscripts consist of quires,
small packages of folded leaves. The scribe would copy the text onto
the pages of the quire, which would later be bound together to form
the completed manuscript. To make sure that each finished quire
ended up in the correct order, the scribe often wrote the first words
of the next quire in the lower margin of the last page he copied.
These are called "catchwords" – the image shows one.
If the catchword at the end of the quire matched the first word
on the next quire, then they were in the correct sequence.
Sometimes, to further help binders put the quires in the right
order, scribes would number them. In the later Middle Ages,
further organization was added to the page by also numbering
the individual bifolia, so as to keep track of their specific location
within the quire. In spite of all this emphasis on location, from
time to time binders still jumbled up the sequence.