Slip joint
A slip joint folding knife does not have a lock mechanism, but relies on a piece of steel along the spine of the handle to act as a spring and hold the blade open or shut - most Swiss Army knives are made in this way. These were also the most commonly made traditional Sheffield folding knife and have remained popular in the UK as they are legal to carry as long as the blade is less than 3” long.
Friction folder
The simplest folding knife of all has no mechanism to hold the blade open and has been found on archaeological excavations dating back at least x thousand years. The knife is simply kept open by holding the protrusion from the back of the blade in line with the handle.
Ferrule lock
The simplest locking mechanism on a knife is a twisting collar on the ferrule which stops the blade folding on the user’s fingers. The Opinel knife is the most common knife using this mechanism:
Back lock
A refinement of the slip joint where the spring along the back of the knife has a hook on it which engages in a notch in the back of the blade when it is opened. The knife is closed by pressing on the spring to raise the hook out of the notch.
Liner/frame lock
Part of the liner inside the knife, or the frame of the knife itself, is spring loaded to engage with the blade and hold it in place when open. The knife is closed by pushing the spring loaded piece out of the way.
