Are the buttons on your shirt on the left side or on the right? There’s actually a pretty easy answer: if you wear women’s clothing, the buttons are on the left side of the shirt. However, if you wear men’s shirts, the buttons line up on the right side. This would make sense if all women were left-handed and all men were right-handed, but because the majority of all people are right-handed, that’s clearly not the reason. So what gives?

Like many old customs, no one’s really sure how the button-side switcheroo came to dominate fashion design. But according one of the more popular theories, it might have to do with how middle- and upper-class European women used to dress, Caitlin Schneider writes for Mental Floss.

During historical periods like the Renaissance and the Victorian Era, women’s clothing was often much more complicated and elaborate than men’s - think petticoats, corsets and bustles. But while rich men often dressed themselves, their female family members most likely had servants to help them put on their clothes, both out of luxury and necessity. To make it easier for servants to button up their employer’s dresses right, clothiers might have started sewing buttons on the opposite side. Eventually, as clothing became more and more mass-produced, women’s clothes kept being made with the buttons of the left, and the design became standard, Benjamin Radford wrote for Live Science in 2010.

That’s one explanation for why women’s clothes button on the left - but why would men’s clothes always button on the right? That particular tradition might have roots in how men once dressed for war, as Megan Garber writes for The Atlantic. Just as wealthy women needed servants to help them get dressed, men’s clothing might have taken cues from military uniforms.

According to the authors of “The Art of Chivalry: European Arms and Armor from the Metropolitan Museum of Art”:

To insure that an enemy's lance point would not slip between the plates, they overlapped from left to right, since it was standard fighting practice that the left side, protected by the shield, was turned toward the enemy. Thus, men's jackets button left to right even to the present day.

Because male soldiers also often drew their weapons with their right hand, building their clothes with the buttons on the right side would have made it a lot easier to adjust and unbutton with their free left hand, Garber writes. But these are far from the only theories that seek to answer this question. Others include the fact that many women breastfeed while holding their baby in their left arm, or that Napoleon mass-produced clothing that was intentionally difficult for women to put on. Historians may never know exactly how women’s buttons ended up on the left, but regardless of its origin this design quirk is now just another sartorial custom.