Three Quarter Swap
In Swinging, the Three Quarters Swap is sex between two couples, where one woman may only have sex with her husband or the other female, but not the other male. So a man can have sex with his wife and the other woman, the other woman and the other male, or the other woman, but his wife may not have sex with the other male.
Hey baby, we're going swinging tonight, but we're only going to do a Three Quarter Swap. I don't want you messing around with any guys besides me!