Slip Joint Springs
Here are a few drawings of slip joint knife springs.
A spring for a single blade, it is pinned in place at the butt. An example of this spring would be the Cowboy and Rancher Pocket knives (Discontinued).
A spring for a blade at both ends. An example of this is the Reverse Congress Muskrat (Discontinued).
Lobster Pen Spring
It is very seldom that you see this spring used.
A spring that has two blades at one end and one at the other. An example of this is the Hen & Rooster Lobster Pen knife (Discontinued).
3 - Blade Pen Spring
Used in boy scout knives such as the Boker Scout Knife.