Folding knives date back at least as early as Roman times. Such knives have some obvious advantages over fixed blade knives in that folding knives provide a particularly compact and safe way to carry the knife in which the cutting edge is not exposed. However, folding knives also have some disadvantages because the fact that the blade must fold makes such knives inherently weaker than a fixed-blade knife. The blade of a functional folding knife must lock into place firmly when closed or opened, but must also be loose enough to permit the knife to be opened easily.
Hold the knife handle in one hand. Make sure that your fingers grip one side of the handle while your thumb grips the other side in such a way that no part of your hand will block the blade from swinging out of the knife handle.
Place your other hand on the blade of the knife in such a way that your fingers can access the nail mark, thumb stud or other opening mechanism.
Pull the knife blade open while maintaining a firm grip on the blade and its opening mechanism.
Make sure the knife blade is completely opened and has been locked into place. Most folding knife blades lock into place using either a lockback or linerlock mechanism. A lockback mechanism is a spring-loaded steel bar that fits into a notch on the back of the knife blade when the blade is opened. A linerlock mechanism consists of a metal plate that slips sideways when the blade is opened and fills the space the blade occupies when the knife is closed. Both locking mechanisms are designed to prevent the knife blade from closing while the knife is in use.
Keep your folding knife clean and well-maintained in order to insure that it opens easily. Dirt and rust can impede the blade's ability to pivot smoothly and can make it difficult or impossible to open the knife. Knife makers have developed a number of different mechanisms intended to facilitate opening a folding knife. Such mechanisms include the nail mark, thumb stud, or a ridge built into the back of the blade.
Folding knives are considered dangerous weapons in many states. Make sure you are aware of any laws and regulations governing folding knives in your state if you plan to carry the knife with you.