The slip joint is the oldest and simplest folding-knife locking concept: it is not a true lock at all, but rather a carefully tuned spring system that keeps the blade either open or closed. A curved backspring—thin, tempered steel running the length of the handle—rides in a shallow notch on the tang. When the blade is rotated past the spring’s neutral point, the spring snaps into the notch and supplies enough resistance for normal cutting tasks yet will yield if deliberate pressure is applied to the spine. Most designs incorporate a half-stop halfway through the arc, giving audible feedback and reducing the chance of accidental closure. Because there is no mechanical obstruction, the blade can be closed with one hand by pushing the spine while pinching the handle; equally, the spring tension prevents it from flopping open in the pocket. Traditional makers such as Case, Victorinox, and Böker still produce millions of slip joints annually, favoring brass liners, nickel-silver bolsters, and natural handle materials. Modern iterations—Spyderco’s UK Penknife, Benchmade’s Proper, and Kizer’s Splinter—use skeletonized liners, CPM-S30V blades, and refined spring geometry to deliver one-hand opening and improved centering while remaining non-locking for legal carry in the UK, Germany, and parts of Scandinavia. The system’s advantages are low weight, slim profile, minimal parts, and near-zero maintenance; drawbacks are limited strength, absence of a positive lock, and the need for careful handling during heavy cuts.
