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A handkerchief code, or hankie code, is a way a person can indicate what type of sexual activity they are interested in. While it is most often seen in the gay community, it is also common within the "other" categories of human sexual expression, including BDSM devotees.
In PBS's Out of the Past several men who were interviewed related that in the 1920s and 1930s, when all gay activity was fully underground, the habit of wearing variously colored breast pocket handkerchiefs to let other gay men know they were out there, and what their preferences were. The colors were mostly limited to "Gay — looking", "top", and "bottom".
Wikipedia mentions that cowboys in the old west would wear their bandanas on their wrist when dancing with other men, in purely platonic situations, to identify who was dancing the male role, and who was dancing the female role. Similar conventions may apply at modern-day contra dances; when there is a surplus of men or women, dancers often take the "opposite" role to fill out a set or square. Such cross-dancers may wear a marker to avoid surprises in the contra line.
 The codes
Of course, as with many things in life, there are no actual rules. "local rules" or "bar rules" often apply, and right butt cheek, left butt cheek, or around the neck can all mean different things at different places and times, so, buyer beware.
But some standards are:
- Black: S&M
- Grey: Bondage
- Olive Drab/Camouflage: Military fetish
- Red with black stripes: Likes bears
- White or checkered: "Safe sex" and "Vanilla sex" (left "top", right "bottom")
- Yellow and Brown: left to the reader's imagination for now