LoveHoney — the makers of the C-string — describe their product as invisible underwear. The website says "C-String has a flexible internal frame that hugs and holds it to the body both securely and comfortably. Your modesty remains safely covered at all times." So, basically, it's a thong without side-straps and is shaped like the letter C.

If you are wearing tight pants or tight jeans, even a thong of g-string can show through, but the C-String in the absence of any waist band means that it eliminates any visible panty lines.