Apply just the right amount of pressure needed for training and correction with Omni Pets Atlas Choke Chain Dog Collar. Heavy-duty and strength tested, the collar is made of Atlas nickel-plated steel for quality thats superior to other collars. Sizes: 2.5mm x 18, 3.5mm x 24, 4mm x 28. Cancer and Reproductive Harm- www.P65Warnings.ca.gov. Size: 2.5MM X 18. Type: Collars.