I did not know that yesterday!

The answer to this one is easy - or is it? -- coins. It has to be coins, right? I scoured dozens of online jeans listings. Pretty much every "classic five pocket design" boasted a coin pocket in the front right scoop pocket. Some of the coin pockets were exposed, some had flaps, some had insignias, but they were all described as coin pockets.

But then I stumbled onto a similar query on Ask MetaFilter and found the real answer -- watches. Back in the day before the wrist became multi-functional, folks carried pocket watches. These were fastened to the end of chains, and stored in special pockets woven into pants or vests.

Levi's 501s, originally introduced in 1890, featured watch pockets. And sure enough, after more snooping around, we found these vintage jeans from the 1970s listed as having "five pocket styling with watch pocket."

interestingly enough, in France it is referred at the "Zippo pocket" - one can only assume that could be a reason for the poor French time keeping, or a reason why they smoke so much.

For more on pockets and their uses, the BBC has a surprisingly interesting essay entitled A Very
Brief History of the Pocket. And this Smithsonian piece details the evolution of pocket watches to wrist watches.