The custom folding pocket knife is known to have originated at least back in the Iron Age. Ancient, small folding knives have been found in areas that were conquered by the Romans, and other areas, dating back to at least 600 BC.
Folding knives were created so that the user could fold the blade in, and carry it easily without having to worry about cutting oneself with the exposed blade. The design slowly evolved until the knives became small enough to fit in a common pocket in a jacket or pants.
Now days, many pocket knives are slip joints, which means that the blade does not lock. However, once the blade is opened it is held in place by tension that allows the blade to fold if a certain amount of pressure is applied.
The lock (lock-blade) knife is a different variety of folding knife that has a locking mechanism that locks the blade in place once it is opened. The lock is then released before the knife can be folded back into closed position. The locking knife improves safety by preventing accidental blade closure while cutting.
The custom folding knife pictured above was created by Jason Story of EEKnives and Woodwork who has a store on JustCustomKnives.com.