The letters written or printed in the tail margin of the first leaf (at least) of each gathering or section of a book, used to guide the binder when assembling the gatherings. Signatures usually run A - Z (or a - z), omitting at times J and V before 1800 due to similarities with I and U. Printers and binders often refer to signatures to indicate a gathering or section itself.
mzh , dg
Kustode; Lagenbezeichnung; Lagensignatur
John Carter and Nicolas Barker, ABC for Book Collectors, 8th edition (New Castle, DE: Oak Knoll Press, 2004).