A woman with a visible thong

See Thong for other meanings.

A thong, tanga or g-string is a narrow piece of cloth or leather that passes between the legs and is attached to a band around the hips, worn as a bikini bottom or as underwear. It is sometimes disparagingly called butt floss, referring to its tendency to slide between the buttocks like dental floss between the teeth.

Although it was worn for decades by exotic dancers, the thong first gained mainstream popularity in South America, particularly in Brazil in the 1970s. In Brazil it was originally a style of swimsuit whose rear area was so thin — often just a string — that it would disappear between the wearer's buttocks.

Previously, this style of clothing had been called a "g-string", a name that was in use at least as far back as the late 1800s. The origin of the word is uncertain: it may have been an analogy to the thickest string on a violin; perhaps it was a euphemistic abbreviation of girdle-string or groin-string; or it may have been a Native American word that simply sounded like g-string. G-strings can be seen being worn in some old photographs.

Many "primitive" tribes, such as some of the Khoisan peoples of southern Africa, wore a similar style of clothing for many centuries, if not millennia, before the current style. Their word for it, tanga, is used in languages such as Spanish to refer to the garment, sometimes specifically the type with cloth through the legs and string around the sides. (In English, tangas are briefs that have full rear coverage but only a waistband at the sides.)

By the mid-1990s, the design had made its way into most of the Western world, and thong underwear became popular. As of 2003, thong underwear is one of the fastest selling styles among women and is also gaining some popularity among men. One advantage attributed to the wearing of thong underwear is that no visible panty line can be seen even under a thin, light-colored, or skin-tight garment. The main drawback to thong underwear is that the fabric must be pulled directly between one's buttocks, which many people find unattractive and/or uncomfortable, but which others actually enjoy. If worn for long periods of time, they can also lead to urinary tract infections in women due to anal bacteria contamination.

Today, there are a number of intermediate styles between full rear coverage and a string rear. A style that has a narrow band of fabric in the rear that just covers the cleft in the buttocks is often called a "Brazilian" rear, because it is often seen at Brazilian Samba carnivals. In recent years the hip hop and R&B music industry has also helped to promote the thong by composing songs about it and featuring artists clad in them. One example of this is the "Thong Song" by Sisqo, which was released in 2000. Many prominent models have also helped advance its pop culture acceptance.