Most people think the small pocket on the front of jeans is the 5th pocket.
Since the 1800's jeans were made with a total of 4 pockets: 2 in the front, a small front right pocket for a pocket watch (high fashion at the time) and a back right pocket.
So which pocket was added?
Starting in the early 1900's denim companies add a 5th pocket to the back left of their reliable blues. Securing them with rivets for added durability and creating the 5 pocket jean we all know and love today.
Until recently, this 5th pocket was little used, mainly giving added-durability as an extra layer and a symmetrical look. Most men would agree the back left pocket of their jeans is the least-utilized of all of their denim. Today, the left pocket is experiencing a rise in popularity as a perfect spot for the modern man's pocket watch - the smartphone.