A seat belt in a car extends from its housing when pulled across the body. It fastens into the buckle with a click. During the drive, it holds position against the wearer. When unbuckled, the belt retracts back into place.

Extension and retraction occur in plain view. The buckle latches visibly. Tension on the belt is felt and sometimes adjusted for comfort.

Within the retractor housing, visibility recedes. A coiled spring maintains steady tension on the belt spool at all times. An inertia reel mechanism tracks vehicle movement through a pivoting mass. Under normal acceleration, the spool turns freely. During sudden stops, the mass shifts to engage a lock, halting belt extension.

These reel operations continue across each journey. The spring tension persists. The inertia monitoring remains active. All occur enclosed in the housing, beyond routine checks.

The seat belt system operates through these visible motions and enclosed reel functions.