A swivel does two jobs. First, it kills line twist — a spinner, spoon, or current-spun bait would otherwise spin your line into a tangled mess after a few casts. Second, it acts as a stopper on slip-sinker rigs: a sliding bullet weight or egg sinker on the main line hits the swivel and stays put while the leader and bait dangle below.
Two styles cover most situations:
- Barrel swivel — two fixed eyes. Tie main line to one end, leader to the other. The standard for Carolina rigs, drop-shot setups, and bottom rigs.
- Snap swivel — one eye + a clip on the other end. Lets you swap lures without re-tying. Convenient but adds a weak link; pros usually skip it on light tackle.
Size by line strength: tiny barrels for 4-8 lb trout setups, mid-size for bass and walleye, heavy ball-bearing swivels for salmon, steelhead, and saltwater. Ball-bearing swivels rotate freely under tension where a cheap stamped barrel can stick — worth the few extra dollars when you're trolling or fighting strong fish.