The handkerchief code (also known as the hanky code, the bandana code, and flagging) is a color-coded system, employed usually among the gay male casual-sex seekers or BDSM practitioners in the United States, Canada, and Europe, to indicate preferred sexual fetishes, what kind of sex they are seeking, and whether they are a top/dominant or bottom/submissive. The hanky code was widely used in the 1970s by gay and bisexual men, and grew from there to include all genders and orientations.

Today, wearing color-coded handkerchiefs (bandanas) is the manner in which communication of desires and fetishes is achieved. Wearing a handkerchief on the left side of the body typically indicates one is a "top" (one considered active in the practice of the fetish indicated by the color of the handkerchief), while wearing it on the right side of the body would indicate one is a "bottom" (one considered passive in the practice of the fetish indicated by the color of the handkerchief). This left-right reality is taken from the earlier practice of tops wearing their keys on the left belt loop and bottoms on the right to indicate being a member of the leather subculture. Bandanas might be worn in the front or back pocket, tied around the neck (with the knot positioned on either the left or right side); around the ankle (when wearing boots or when undressed); or on other parts of the body.

There is no universally understood color code, and regional codes vary widely.