The first pocket knives in history were used as early as Roman antiquity, but these did not yet have locking mechanisms and were held open solely by friction on the inside of the handle halves. Today, these simple knives are known by the English term friction folder. To make pocket knives safer to use, numerous mechanisms were developed over the centuries to lock the unfolded blades in place. We would like to briefly explain the function of the most common constructions used today.