Handkerchief code or hanky code (also known as bandana code and flagging) is a way of indicating, usually among gay male casual sex-seekers or BDSM practitioners in the leather subculture in the United States, Canada and Europe, whether they are a top or bottom, and what kind of sex they are seeking, by wearing color-coded handkerchiefs (bandanas), usually in the back pocket. The hanky code was widely used in the 1970s as a "secret" gay code that was intended to go unnoticed by non-gays, but general awareness of the code has gone increasingly mainstream.

There is no universally-recognized, "official" color code, and regional variants of the code exist. However, there is widespread consensus on the meanings of a handful of colors, based on decades of precedent in the gay communities of many different countries -- such as yellow for watersports; red for fisting; and black for SM. Also, generally speaking, it is understood that a hanky displayed in the left-side pocket signifies a preference for taking the "top" or "dominant" or "Master" role, while the right side represents the "bottom" or "submissive" or "slave" role.

Today the bandanas are worn almost exclusively in the rear trouser pocket, on the left side of the body for tops and on the right for bottoms, just as tops wear their keys on the left belt loop and bottoms on the right. In the past bandanas were also worn tied around the neck (with the knot positioned on either the left or right side); around the ankle (when wearing boots or when undressed); hung from the left or right side of the belt, or on other parts of the body.

While, for the most part, hankerchief coding is obsolete, the colours and representations from this method of communication are often modified for other purposes, such as the creation of pride flags and symbols.