For those of you who don't know, a cravat is a type of necktie worn in the 18th and early 19th centuries. For purely illustrative purposes, here's a picture of Colin Firth modeling a cravat.
The cravat stayed popular until about 1692, when it was replaced by a similar necktie called the Steinkirk. The cravat made a comeback in the 1770's, thanks to the maccaronis (yeah, those guys). They were popular once again until they morphed into neckties after 1815. They were commonly black, replacing the white linen or muslin ties of the Regency. Here's a handy tutorial on how to tie a post-1815 cravat/necktie.
|A sculpture of a cravat by Grinling Gibbons. It appears to be made out of a combination of muslin and lace or netting. Source.|
During the Regency, the way a gentleman tied his cravat was one of the only ways he could distinguish himself from other gentlemen in dress without being labeled a fop. A veritable smorgasbord of knots and ties existed.