I'm reading The Universal History of Numbers by Georges Ifrah a few pages at a time. If you were like me, you had an elementary school teacher who reprimanded students who used fingers to count; but really it's a good way to reinforce understanding. Moreover, finger-counting has been used in remarkably complicated systems for calculations by many cultures over millennia. Ifrah illustrates one discussion with an image based on this early Renaissance painting of the 6th C philosopher and mathematician, Boethius, which is part of a fresco on the north wall of the Ducal Palace in Urbino. The portrait set me thinking that it would be worthwhile to pay attention to Finger Counting and Hand Diagrams in medieval illuminations in order to read them correctly.
The picture also makes me think that one of the medieval systems could be used in historical fiction or one of those mysteries set in the present that depend on old books or manuscripts for the crucial clue. Or a writer could invent or adapt one for a coded thread in a speculative fiction. In any case, the portrait is a reminder that paying attention to details can yield all sorts of surprises.