Not all button-up shirts are button-downs. Here's why.

Men likely have a closet full of these — even if they don't exactly know what to call them.

You probably refer to that shirt with a collar and buttons along the middle as a "button-up," and you wouldn't be wrong, but is it also a "button-down?" Let's break down the difference.

The main thing to know? Button-down shirts have extra buttons so you can fasten the collar and keep it in place. Take a look at "Modern Family" star Ty Burrell's collar here.

Brendon Thorne / Getty Images for Walt Disney Studios
See the buttons on the points of Burrell's collar? That means his shirt is a button-down.

"A button-down is a button-up, but a button-up isn't necessarily a button-down," style expert Megan Collins of Style Girlfriend, a website for men's fashion, tells TODAY. "In the way that an apple is a fruit, but not all fruits are apples."

RELATED: Here's why men's and women's shirts button on opposite sides

Here's a tip to help you remember: You can button down the collar on a button-down.

Not all button-up shirts have those extra buttons, though. See actor Alexander Skarsgard as an example.

Getty Images
Skarsgard shows us what a button-up looks like. Take note of the button-free collar.

The reason behind button-downs is rooted in function. The shirt was created for polo players in England to keep their collars in place while they rode horses, Collins explains. The style stuck and today it's considered a slightly more casual version of a dress shirt.

RELATED: Here's one reason you shouldn't wear an untucked shirt to the office

Funny enough, what's commonly known today as polo shirts — think Ralph Lauren — don't actually have those extra buttons. Those were made for the tennis court, not the polo field, Collins says, where there's "not as much bopping up and down" as there would be on a horse.

So go ahead, show off your knowledge the next time you shop for shirts — you'll finally know exactly what to call the item you're buying.