I first saw this kind of trousers at my job, sorting costumes, and I thought it was pretty smart. They are called ‘fall front trousers,’
‘drop front trousers,’ or sometimes ‘flap pants.’ Zippers were not in use in the nineteenth century, and having a button front closure on trousers might have been seen as uncomfortable or not elegant enough, the trousers were closed with a ‘flap’ which buttons on the sides or top. Under the flap, the waistband has a front closure so you can open the flap without dropping trou (convenient, convenient.) The pockets are also located under the flap. Trousers like this were worn from the French Revolution onwards (1790s), around 1840 the centered trouser closure was introduced but for a long time the two styles existed simultaneously.
Picture credit to vintagetextiles.com
My very favourite emperor Napoleon Bonaparte was a big fan.
Now you might think, I want one of those! Luckily, Marc Jacobs thinks they’re very sexy too: