Handkerchief code
The handkerchief code (also known as the hanky code, the bandana code, and flagging) is a system of color-coded cloth handkerchiefs or bandanas for non-verbally communicating one's interests in sexual activities and fetishes. The color of the handkerchief identifies a particular activity, and the pocket it is worn in (left or right) identifies the wearer's preferred role in that activity.