Knives held by spring, not a lock
A slip joint is a folding knife without a lock; Kershaw defines a slipjoint as "a knife with no lock." Case says non-locking knives rely on spring tension to stay open, and Buck says a spring generally holds them open. Kershaw says many traditional slipjoints have a steel leaf spring and others a ball detent.
Nothing locks an open slip joint blade in place. Case says non-locking knives "rely on spring tension to keep the blade open", and Buck says "generally a spring holds the blade in the open position."
The holding mechanism differs by design. Kershaw says many traditional slipjoints have a steel "backspacer that acts as a leaf spring", keeping tension on the blade as it opens and closes and biasing it toward closed when shut. The site's glossary calls that part the backspring. Kershaw says other slipjoints use a ball detent, a small steel ball on the frame or liner that fits an indentation in the blade.
Pressure on the spine is what closes a slip joint: Böker says the knife is closed "by putting pressure on the spine of the blade to overcome the resistance of the spring". The same force applied by accident during a cut can fold the edge toward the fingers, and Case says non-locking knives "should always be used with care." The safest way to close a folding knife walks through guiding the blade home.
Böker says multi-blade folders are one of the main applications of slipjoints, because the mechanism allows several blades side by side, and Case's own store files its Medium Stockman under the lock type "Folding Slip-Joint". The stockman and other multi-blade designs each get a line among the traditional pocket knife patterns.
The grid on this page pairs a traditional Böker Toothpick with modern designs: the Böker Plus Atlas, the Kershaw Federalist and the Case Highbanks, each named or described as a slip joint in its listing.