Also known as: 腰布, 前掛け
Any of several ancient styles of garment designed to cover the genitals. Usually made from a folded and tied length of cloth, sometimes held in place by a belt. If it has a dangling length of cloth hanging in front and/or or behind, that part is called, for Danbooru's purposes, a Pelvic Curtain.
Fundoshi are a range of Japanese loincloths. The loincloth's modern replacements are Panties, Briefs, and Boxers.
Both Loincloth and Fundoshi nowadays are used as festival clothing (usually with Happi, or alone), as a Swimsuit (when worn at the Beach) and occasionally, as Underwear (usually worm under Kimono, or Japanese Armor etc). Make sure to use the appropriate tag when tagging them.