Have you ever gotten your thumb stuck in there due to its teeny-tiny smallness? Well, wonder no longer.
It's a watch pocket, originally for men who wore pocket watches and needed a protective place to store them. Though, as Levi's points out, the pocket has also served many other purposes over time, from watch storage to coin hoarding.
The burning question arose this week after curious minds re-discovered a Quora thread in which users have discussed this mysterious fifth pocket for years.
The Levi Strauss blog confirms their conclusion: "The first blue jeans had four pockets—only one in back and, in the front, two plus the small, watch pocket. This extra pouch has served many functions... coin pocket, match pocket and ticket pocket, to name a few."
Not only is the pocket extremely useful for holding tiny trinkets, it is also is loved by denimheads for the faded and worn nature it takes on over time.
The pocket appears on the oldest pair of jeans in the Levi's archives, which date to about 1879.
The more you know.