Buttons.

That's why Levi's 501 jeans have buttons. It's a deliberately retro reference.

My answer is going to be rejected for /r/askhistorians by a bot because it's too short. So I'll make it a bit longer. Fun fact: almost all zippers are made by the same Japanese company, YKK. The zipper debuted at the 1893 World's Fair in Chicago, which is the same place where the Ferris Wheel and Pabst Blue Ribbon debuted. That is, Pabst claims it won a blue ribbon for quality at the fair and changed its name as a result. Others are dubious about the veracity of the story.

People were initially slow to adopt the zipper in their pants, with many thinking it was inferior to the button. Mensware style guides didn't start advocating it instead of buttons until the 1930s. It wasn't until well into the 20th century that it was adopted widely in clothing.