Why do jeans have a tiny extra pocket?

Answer

The small pocket dates from the 1800s.

Context

The small pocket first appeared on riveted work trousers designed by tailor Jacob Davis and sold by Levi Strauss. Their U.S. patent of 1873, titled 'Improvement in Fastening Pocket-Openings' (No. 139,121), covered the use of metal rivets at stress points, such as pocket corners, to stop tearing. The rivets were the invention; the small pocket was a feature of the early design, and Britannica notes it was intended to hold a pocket watch.

According to Britannica's account, the company did not include the small pocket in mass-produced blue jeans until the 1890s. The target customers were laborers such as miners, railroad workers, carpenters and farmers. A watch carried in the main pocket could be crushed or scratched during heavy work, so a separate small pocket, set inside the larger one and sometimes called a watch pocket, gave it protection.

The name has changed over time. Levi's called it a watch pocket in early catalogs, and later it was often referred to as a coin pocket, a ticket pocket or a 'fob' pocket, depending on what wearers put in it. When wristwatches replaced pocket watches in the 20th century, the pocket stayed as a standard part of the pattern, even though it is awkward for modern phones and most items.

Suit trousers rarely had it because gentlemen's waistcoats and jackets already had pockets for watches. That difference explains why the feature survives mainly on casual work-derived garments: it is a trace of the clothing's working-class history, not a recent fashion idea.