A credit card knife is usually a very thin knife that is the shape and size of a credit card, either when folded into a knife shape or unfolded for storage. It is designed to be carried in a wallet along with regular credit cards. Some credit card knives can contain other small tools, such as tweezers, or toothpicks. A ballpoint pen knife is generally a pen with a concealed knife inside, which can be used as a letter opener or as a self-defense weapon. Knives with locking blades, often referred to as "lock-blade knives" or "clasp knives", have a locking mechanism that locks the blade into its fully opened position. This lock must be released in a distinct action before the knife can be folded. The lock-blade knife improves safety by preventing accidental blade closure while cutting. It is this locking blade feature that differentiates the lock-blade knife from either the "peasant knife" or the "slipjoint" spring-back knife, although locking knives tend to be larger than either of those types as well, due to the difficulty of putting a lock into a smaller knife frame, the greater chance of doing more forceful kinds of work with a larger knife, and the cost of a locking mechanism is a smaller percentage of the total cost than it would be on a smaller knife, which are generally expected to be considerably cheaper, although this varies a great deal depending on maker and market. Lock-blade knives have been dated to the 15th century. In Spain, one early lock-blade design was the Andalusian "clasp knife" popularly referred to as the navaja. Opinel knives use a twist lock, consisting of a metal ferrule or barrel ring that is rotated to lock the blade either open or closed. In the late 20th century lock-blade pocketknives were popularized and marketed on a wider scale. Companies such as Buck Knives, Camillus, Case, and Gerber, created a wide range of products with locks of various types. The most popular form, the "lockback knife", was popularized by Buck Knives in the 1960s, so much that the eponymous term "buck knife" was used to refer to lockback knives that were not manufactured by Buck. The lockback's blade locking mechanism is a refinement of the slipjoint design; both use a strong backspring located along the back of the knife handle. However, the lockback design incorporates a hook or lug on the backspring, which snaps into a corresponding notch on the blade's heel when the blade is fully opened, locking the blade into position.