These days, folding knives are becoming increasingly popular because of their portability and all-purpose usability. Very often folding knives are equipped with a kit of additional tools. Early models of the mid 19th century featured a blade hinged to a handle. This type of fastening is still being used by certain manufacturers but its durability is doubtful. Models with a hinge with an extended spur on a blade heel were developed, and these knives were much more durable than the early models, but less convenient to use. The most up-to-date blade fastening type is a flat spring to fix blade in both positions. Ballisongs are another knife type in which the blade is hidden between handle halves. Hinge knives feature a handle that folds into two and is fixed with a pin.
There is a wide variety of locks used to fix a blade, and every manufacturer tries to develop his own expertise in this field. In lever locks, the lever is used to unfix a blade; whereas, in line locks, a special spring platen is used to fix the blade heel with a particular lug. Compressive locks are equipped with a cross pin to fasten the spring. There are also different pin locks in which a part of a handle is used instead of flat spring (the pin can be situated in different planes).
Blade shape of folding knives can vary greatly from classical straight blades to claw blades designed primarily for self-defense. Often a blade root has a shape combining hollows and trenches of different sizes intended to increase cutting characteristics and to inflict lacerated wounds.
Very often, special purpose folding knives are equipped with additional tools such as a belt poleaxe, glass-breaker, saw, screwdriver, corkscrew, awl, skinning blade, etc. Sometimes the difference between general-purpose knives and special-purpose knives is very small, so some knives are equipped with some special tools in addition to a standard traveling toolkit.