Intercrural sex, a.k.a. thigh sex, is a type of sex that can be performed between couples where at least one of them has a penis. The way it works is, the “receiving” partner squeezes their thighs together, and the “penetrating” partner thrusts their penis between those thighs. As a sex act it has a surprisingly long history, or maybe not that surprising, if you think about it.
In Ancient Greece it was an extremely common sex act as the predominant way that male-to-male sex happened; it was even represented on vases.
It was also pretty well-known and common in early modern Europe. Through the trial of a sitting MP in 17th century England, it was widely associated with male-to-male sex, but it was also a reasonably common sex act for opposite-sex couples as well, because it was a way of having sex that carried a waaaay lower risk of pregnancy or transmitting STDs in an era before condoms existed (hence why I earlier said it’s not that surprising, in retrospect, that this act has such a long history).
In traditional Zulu culture intercrural sex also has a pretty extensive history. It was, apparently, encouraged among young people as a way of giving them some “sexual socialisation” before they actually settled down to have a family with a specific partner. Some Zulu people still have/encourage this form of sex today, but with Christianisation many others advocate abstinence for young people, instead.
In more modern times, intercrural sex has also been promoted in Africa as a way of having sex that carries less risk of transmitting HIV/AIDS.