the lock

I use generalizations in this and other posts. A lot. There are exceptions, which is going to be the focus of this blog after all the introductions and explanations are done.

The lock isn't a step into itself. In fact, it's not a step at all, it's a feature. However, I felt it necessary to describe it here as it's an integral part of the closing step. Once a blade is opened, something needs to keep it open.

With classic knives, like Case and Swiss Army, it's a detent and friction. Nothing needs to be done before closing; you just push the spine of the blade to start it closing.
Modern folding knives have a safety, a positive lock, to keep the blade open to prevent it from closing (on your hand while holding the handle). The two most common are the liner-lock and the lock-back.

The liner-lock, or it's variant the frame-lock has a pre-bent piece of metal that moves sideways into a space inside the handle, behind the blade, created when the blade is fully open. It needs to be pushed out of the way to close the blade. Think of the back of the blade being D shaped. When fully opened, the flat part of the D mates with the flat shaped end of the liner-lock. When the blade isn't fully open, the round part of the D prevents this mating, because the blade rotates around the pivot.
The lock-back has a spring loaded bar of metal that snaps down into a space created when the blade is fully opened. This bar rides along the top, or spine, of the handle. To disengage, you push the far end of the bar down, lifting the other end out of the space (like a seesaw), unlocking the blade.