Can you possibly guess what was the best (by far) consequence of Emperor Napoleon's worse nightmare?
Yup, you guessed: the metric system.
Those unblessed with traditional measurement systems (which, well, it's just the USA, Libya and Burma) know perfectly well what I mean. But, well, most countries still keep some traditional measurement units in common-day use, although not in any formal written texts, except, maybe land surfaces. UK and other Common Welth countries still use pints, pounds, and so on, but...
Anyway, when writing Historical Fiction, when we have to describe any measure we know we are in deep trouble. How much was a talent worth? A gold talent? A silver talent? And where? In Persia, in Athens, in Alexandria? In Rome?
And, well, armies walked pasasangs, stades, miles (roman miles? nautical miles? which ones of the many kinds of miles?). And here we have the fun of weights: grans, talents, grams, gallons, tons --yes, some look like volume units, but they were pretty much all fussed up and you never exactly know when a primary source is using one or another, and if when describing other peoples' units, they use their meaning or the author's meaning or which other one!
Now I'm exploring the fun of lists and lists of food supplies, medicine supplies, water and wine and vinager volumes, number of cannon balls, with different weights, diameters, for cannons (and other strange-named weapons, like falcons, falconettes, culverins, and others) for the different type of ships (which, well, was also an exercise of imagination, becasue you could find ships described as urcas from almost 900 tons to 60 tons, and they are all tucked together... !!!).
BTW, how many tons? Castilian tons, volumetric tons? Similar, but not quite...
And the trouble, the fact that you have to manage about 130 Spanish ships and more than 200 English ships! So many facts, names, data, measurements, crew sizes, number of cannons, displacements, sails and whatnots!
I can firmly say that measurent (and monetary systems) are the suckiest aspects of writing Historical Fiction.
But, somehow, we'll manage.