Well, almost contemporary with JA .....;-)

From Francois Boucher's La Toilette 1742.

Here is a close-up of the *ahem* demi- mondiste and her silk ribbon garter which she ties over her knee.:

Other famous picures of 18th century ladies garters can be seen in works by Hogarth : notably A Harlot's Progress : plate 4 of 6( 1732) and in The Rake's Progress; the Tavern Scene..

Basically a garter was a band of leather or fabric tape that held up your stockings. Women usually gartered their stockings above the knee, as shown above, although they sometimes gartered them below the knee. Woollen garters had a little more "give" and could be easily and cheaply made at home.

When using a fabric tape, it seems from the evidence of these pictures, that it was generally long enough to wrap twice ,which would distribute the pressure better.