Fishing swivels consist of a barrel with freely rotating rings at each end. Used as a join between main line and rig or (snood) hook link, they greatly reduce line twist.
How to tie a swivel
Fishing swivels can be tied to any type of line. Whenever using monofilament, copolymer or fluorocarbon fishing lines, a Uni knot is strong and will not slip. For braided main lines a Palomar knot is very strong and easy to tie. Both of these knots are in my 5 Good Fishing Knots article.
Fishing swivels – types
Rolling swivels with round eyes are the most popular swivels used in freshwater fishing. Available in a range of sizes from 2 to 14, with 14 being very small and 2 being big. Size 8 is common in Carp fishing with many rig and feeders using eights. For light ledgering and general silvers fishing, size 12 is a common small size to use.

Diamond eye swivels are designed to be used with fishing line attached to both sides. The diamond shape of the eye ensures the line is always on the swivels axes of rotation. Altogether diamond eye fishing swivels are best for reducing line twist.

Double rolling fishing swivels are used for spinning and lure fishing where line twist can be extreme. Two rollers together will rotate more freely than a single swivel, ideal for Mepps and Toby type spinners.

Rig swivels have a round eye on one side and a diamond eye on the other. The round eye is used on the rig side. The hook link, or whatever, is attached to the diamond ring side.

Quality
Please don’t make the mistake of using cheap low quality swivels. If the swivel fails you won’t just lose the fish, you will lose the lure or rig too. A few quality swivels can cost as much as a bag of cheap ones, but one lure can cost much more. Quality swivels often state their strength on the packet, very helpful when tying a rig.
Conclusion
For general use, round eyed rolling swivels are popular. Diamond eyed swivels are for optimising the swivels anti twist properties. Rig swivels are ideal for rig construction where line is attached to the diamond eye side. Finally, double barrelled fishing swivels are great for spinning. Always use well made swivels rated by size and strength. Use a Uni knot with nylon lines and the Palomar with braid.
Best of luck.
Related
Best split shot for float fishing
How I put split shot on fishing line