1" Steel Clamps with Hex/Allen Wrench. Fantastic collars. I originally bought four dumbbells for two of my weights. I just bought eight more for my other 1 inch dumbbells. Excellent quality. I tightened the screw with a hex wrench and it seems very tight. Fits perfectly. Don't make the same mistake I did and don't buy those cheap 1 inch plastic collars. Solid steel 1" clamps instead. The picture shows an example of cheap plastic bottles that you wouldn't want to buy. In the picture I also left the swivel collar because I had space, but I'd feel very comfortable if I only wear this collar when I don't have enough room on the bar.