The X-like letter in the title of this post is actually the lowercase form of the Greek letter ‘chi’ (the uppercase is X). It earns a place in our A to Z of Books for its role in the collation of printed books: that is, recording all the leaves and gatherings (or quires) that make up the physical volume, as explained in our A is for Alphabet post.
You will see the note ‘Signatures’ in many of the newer catalogue records for our early printed books, for example this 18th century volume from the library of the Liverpool Royal Institution:
Signatures: *⁸ ²*² **⁸ A-I⁸ K¹² [chi]⁴ L-Ee⁸ ²A-B⁸.
For many books, the letters of the printer’s alphabet (excluding J, U and W as being easily confused with I and V) are sufficient to describe the book in hand, but sometimes it is not so straightforward, which is where Greek letters come in. It is common to find these spelled out, as in the example above, instead of using Greek characters which may not be accessible across all devices.
But what do they mean? It is common to find books in which not all the gatherings are signed (that is, marked with, for example, A, A2, A3… at the foot of the leaves in the folded sheet), but there isn’t an obvious gap for them in the alphabetic sequence. So, unsigned leaves at the start of the book, before gathering A, are indicated by the Greek letter π [pi] and unsigned leaves elsewhere are given the signature χ [chi].
And like all oddities in early printed books, looking at the physical volume may reveal what was actually going on as the book was printed and why these unsigned leaves are there.
References and further reading:
Karen Attar, “Collational formula” in Michael F. Suarez, S.J. and H.R. Woudhuysen eds., The Oxford Companion to the Book, 2010.
Erin Blake, Signature statements in book cataloging The Collation. Research and Exploration at the Folger (blog)
Philip Gaskell, A New Introduction to Bibliography, 1995.