In the middle ages, would leggings or tights most often be a one piece garment or just long socks? By the sixteenth century, did most men in England and France wear trousers?
My knowledge doesn't go back as far as their invention, but there's a great deal of discussion over the ancient usage of split leg garments in the other links already provided. As for trousers as we know them today, they begin to appear around the early 18th century. Prior to that you have hose, a fitted full leg garment that is more like a pair of non-stretch tights that have two separated legs. By the mid 16th century you see things like pumpkin hose, or what will develop into breeches in the late 17th century. Essentially a separation of the leg covering to sections above the knee and below the knee.
Trousers themselves start out as a sailors garment. A very common runaway ad will read that they wore a "sailors jacket and trousers", a dead give away as to where they came from. Around 1750 trousers begin to show up more commonly in written references like court documents, without reference to sailors. Used mostly by the laboring class or military (gaiter trousers, for example), they really weren't accepted to be "fashionable" until the very late 18th century. While there is a lot of speculation that the French Revolution and the Sans Culottes (those that did not wear breeches, but trousers) caused this shift, it's very likely it would have occurred regardless. There was already a strong trend for country dress than began occurring in the 1780s with chemise gowns and other, almost nostalgic, clothing styles. While British and French fashion were closely tied, one did not ever wholly change the other. An exchange of ideas constantly occurred and each made the concept their own. To be honest, with as much distaste of French fashion as you see in British satire of rich and poor, it's hard to believe that they would take up trousers solely because of the revolutionaries trends. Those that did take up French fashions during this time were even mocked. The British are still wearing breeches above all else in the 1790s, trousers becoming more common for day wear in the 1810s. It isn't really until the 1830s that breeches fade out (with the exception of court wear) and trousers are accepted in most situations, formal or casual, in both England and France.
hi! You'll find all kinds of contrasting answers in these threads, which I take to mean that trouser usage changed not only through time, but also differed across geography and between social classes. Hopefully one of the fashion historians can sort this out, but meanwhile, check these out: