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You can wear different coloured (or made of different materials) hankies in your left and right pockets to indicate sexual preferences. The core colours are mostly agreed on, but there are some huge lists which include very specific things.

One major problem with the codes was remembering (or agreeing) on which pocket was which - is a white hanky on your left or his' left that indicates you want to get jerked off?

Some of the local attempts at hanky codes are documented in PeanutButterAndJamSandwichTwelve.

Here are some canonical liists of HankyCodes:

This is one part of MeetingPeople and CruisingProtocols