Fashion conventions, which were previously restricted to men and women, are becoming more fluid. However, there is one slight difference between men’s and women’s clothes, which some historians believe is a centuries-old custom rooted in gender injustice. When you next button your shirt, take particular attention to the placket, which is the strengthened band of cloth that connects the button and buttonhole. Now, closely inspect the clothing of someone of the opposite gender and see if you can tell the difference.

Generally, men’s buttons are on the right, whereas women’s are on the left. Though experts have made different theories, the exact origins of the unusual opposite-button design have been lost to time. One of the most widely accepted interpretations dates back to a time when wealthy women did not dress themselves. Because most people were right-handed, the buttons were placed on the left to make it easier for the maid to connect them. Upper-class women often had maids to help them get ready.

“When buttons were invented in the 13th century, they were, like most new technology, very expensive,” Melanie M. Moore, founder of the women’s fashion brand, told Today. “Back then, wealthy women didn’t dress themselves; their lady’s maid did. Because most individuals were right-handed, someone standing across from you had an easier time buttoning your dress.”

Males, who frequently dressed themselves, wore buttons on the right to accommodate right-handed persons. According to other experts, men’s clothing was designed for fighting. Historically, many men carried weapons (such as swords or pistols) and need convenient access. According to Chloe Chapin, a fashion historian, having buttons on the right side made it easier to unbutton jackets or coats while keeping the dominant hand free for action.

“I think it’s important to question which time period we’re talking about,” Chapin, a Harvard PhD student in American studies, said. “But as a general rule, many elements of men’s fashion can be traced back to the military.” The left-side buttons remained because, of course, women’s clothing did not consider combat usefulness.

Masculine twist. Another point advanced by Chapin was that women’s clothing began to include more masculine elements throughout the 1880s. “It was fashionable for women’s attire to resemble traditional male styles. However, it was unlawful in many locations to dress like a man in public, so having a different buttoning pattern may have proven that you were wearing a female clothing,” the historian explained.

Gender inequity. In “Man and Woman: A Study of Secondary and Tertiary Sexual Characters,” published in 1894, British sexologist Havelock Ellis observed that women’s clothing buttoning from right to left indicated that they were perceived as less physically strong than men. He claimed that men, unlike women, had the “rapidity and precision of movement” that women lacked due to poorer motor skills.

‘Button differential.’ People are only now becoming aware of what The Atlantic calls the “button differential,” despite the fact that this little fashion detail has been hidden for generations. According to the journal, it “is a relic of an old tradition that we have ported, rather unthinkingly, into the contemporary world.” The online community is likewise taken aback.

One Reddit user writes, “I was today years old when I learned that women’s shirts have buttons on the left side!” And a second replies, “I *just* discovered that shirt buttons are on the left for women and right for me. Wtf!!!!” Others expressed their thoughts on the centuries-old tradition that still dictates fashion today, which is riddled with gender discrimination.

“Ah, the big gender split – because evidently, centuries ago, women required assistance getting dressed, whereas males were expected to fend for themselves. “Fast forward to today, and we’re still tying up old traditions,” tweets one person. A second Redditor adds, “Zippers, too, usually!” The majority of clothing companies continue to follow the left-for-women, right-for-men rule, while some brands are now questioning gendered button placement.