Catchwords are words which are usually written on the bottom margin of a page which repeat the first word on the following page. Here is an example from Trinity MS R.3.60. Note the catchword ‘singula’ on the bottom of the verso (left) page which is the first word on the recto (right) page.
In MS B.15.25, the catchwords are all contained within grotesque drawings including a man spearing fish,
Why was this done? It helped with the correct order of leaves of paper or quires during binding. Books were formed by binding a number of quires together. The collation is the description of the way in which a book was bound. So, for example, the collation of B.15.25 is described as follows:
18 , 28 , 38 , 48, 58 , 68 , 78, 88, 98, 108, 118 , 128, 138, 146, 156.
In this instance the manuscript was made up of 13 quires of 8 leaves and 2 additional quires of just 6. The number of leaves in a quire can vary. The catchwords occur on the last verso of each quire to link to the first recto of the following quire.
Folios 47v-48r illustrated below is the point where quire 6 ends and quire 7 begins.
The study of the way in which manuscripts are bound and the physical structure of a book is called codicology and can tell the user much about the origin and production of a text.