Underkläder: En kulturhistoria is a Swedish book about the history of undergarments. A surprisingly large part is dedicated to the 18th century, and it’s a very interesting read. There’s quite a lot of pictures, and the text is very entertaining with lots of interesting quotes.
There’s a quote from the 1760s, from a fictional letter in a magazine, that I think gives a lot of insight to the question whether garters were bound above or under the knee. I’ve always figured both things must have been practiced while some people seem to strongly believe they were tied under the knee. From the quote it definitely seems like both were practiced, above the knee being the desired style, and under the knee the most practical. My translation from Swedish:
He reminded me, that beautiful legs are spoilt if the garter is tied under the knee.