The Zipper — How Two Rows Become One
Inside the Y-Channel Slider & Hook-and-Hollow Interlocking Tooth Mechanism
Invented by Gideon Sundback, assigned to Hookless Fastener Company (Filed Aug 27, 1914; Granted Mar 20, 1917). Smithsonian Institution Specimen NMAH-860486.
Drag the slider handle vertically or use the keyboard buttons to witness how the interior Y-channel forces offset teeth into positive locking engagement.
Closed & Interlocked
All alternating scoop teeth are nested projection-into-recess. Longitudinal and lateral forces are held in equilibrium by tooth shoulders, preventing unzipping under tension.
Component Anatomy & Functions
The heart of Sundback’s patent: a dual-flanged chamber with two angled upper entries converging into a single narrow throat at the bottom.
A central triangular divider between upper channels. In the downward stroke, it drives between interlocked teeth, forcing the projections out of their hollows.
Clamped onto corded textile tape edges. Each metal tooth features an upper convex projection (nip) and a matching lower concave depression (socket).
Woven fabric ribbon with an edge cord that provides an anchor for the crimped metal tooth jaws.
Prevents the slider from running off the tapes and holds the initial tooth pair in aligned phase.
Provides mechanical leverage for manual operation; in modern variants, houses a spring-loaded locking pin.
Kinematic Theory & Angle of Convergence
The zipper does not push teeth straight together. Instead, Sundback’s genius was introducing the teeth at an angle. Entering the convergent channel tilted relative to one another, the convex projection of tooth A enters above the socket of tooth B. As the tape flattens into the exit throat, the teeth rotate into horizontal alignment, trapping the projection securely in the socket.
The Opening Separation Sequence
When moving in reverse, the central diamond wedge acts as a dual inclined plane. It forces the joined teeth apart laterally before they can transmit tensile stress to the tape, unlocking each interlocking pair sequentially with minimal friction.
Disclosed Visual Simplifications
To preserve 60 FPS performance and clarity across mobile and desktop, this interactive cutaway models metal teeth with classic Sundback cup-and-cone contours. Microscopic textile weave crimp tolerances and automatic cam locks have been abstracted.
Historical Context & Attribution
Primary Reference: US Patent 1,219,881 A (Gideon Sundback, 1917). Known historically as "Separable Fastener No. 2", this breakthrough eliminated previous hook designs and established the modern zipper standard.