G-string is a designation for men's / women's underpants that have no buttocks and usually only a small triangle or just a rope. G-string underwear is so called because of the element of string / thin straps that are at the waist / pelvis when worn.
The Thong underwear model is the same as the G-string but differs only in the width of the waist / pelvic part of the pants so there is no string / string element.
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From @soniagirl