In the 2015 film Sisters, Kate McKinnon saves the day by whipping out a staple of the lesbian lifestyle: a collection of carabiners.

For some reason, lesbians love carabiners, the square-ish metal keyrings that loop their belt loops. Carabiners have become just as much a stereotype as flannel shirts, pixie haircuts and short fingernails.

But why?

Historically, lesbians have always been more likely to hold jobs.

In the early 1900s, while heterosexual women had husbands they could rely on to work and be the breadwinner, lesbians never had that option. They were forced to work in order to support themselves, and having a key ring was often a practical necessity for blue-collar jobs.

Butch lesbians were often forced to work blue-collar jobs.

Until very recently, women who dared to work outside the home were caged into “feminine” jobs such as being a secretary. Butch lesbians, needless to say, did not fit those roles.

Masculine-presenting lesbians, therefore, got jobs where they could get them – jobs that did not require them to adhere to traditional gender roles. These jobs were often as mailwomen, mechanics and janitors, which all require easy access to a wide variety of keys.

Key rings became a secret code to determine sexuality.

Soon, women who wore carabiners noticed that other women wore carabiners and that those women were usually queer. A secret code evolved. Lesbians wore their carabiners on the right side if they were bottoms and on the left side if they were tops.

Alison Bechdel brought the key ring to a new generation.

In her bestselling memoir/sold-out Broadway musical Fun Home, cartoonist Alison Bechdel describes the joy of finding another carabiner-clad woman. The song “Ring of Keys” hit home for many lesbians who grew up in the ’80s and ’90s, and helped introduce contemporary “baby gays” to the age-old carabiner tradition.

Straight women stay away from carabiners.

Straight women have co-opted many aspects of lesbian culture, from worshipping Ruby Rose to getting undercuts to even wearing flannel. But the carabiner tradition has been around for decades, and straight women have yet to get on board with clipping a square metal key ring to their belts. It’s hard to make that stylish. So for now, our carabiners are safe.

Learn more at the Slate’s carabiner exposé.