Our friend Tony Campbell has put together a Web page on cartographic chronograms. But what, you may ask, is a chronogram? In a nutshell, it’s a date encrypted into a sentence or inscription. Tony’s short explanation suffices very well:

A chronogram is a sentence or inscription in which specific letters (M,D,C,L,X,V,I), capitalised and interpreted as Roman numerals, have their values added up so as to give a hidden date, e.g. ‘LIncVIt In Isto MonasterIo reLIgIosVs fr. LanDeLInV s bIeheLer IbI professVs’ (= 1781).

(There’s also the inevitable Wikipedia entry.)

But what does this have to do with maps? From Tony’s e-mail to MapHist, where he announced this project:

Chronograms are ‘fun’; indeed, along with acrostics, crosswords, and so on, they are kept alive today as ways of stretching the brain, and particularly linguistic agility. But, for cartographic historians, they can another purpose. … [M]ost of the maps concerned are dated ONLY by their chronograms (though remember that the chronogram’s year may not always be a publication date). In some cases the chronogram has pointed to a lost prototype.

Tony’s page lists 19 known examples of chronograms on maps.