Pro Quality Strong Chain Collars for Dog Training and Walking Ultra strong, great quality Chain Collars feature rugged electronically welded steel construction, for the smart training solution that's built to last. High quality Steel. Thicknesses are as follows: Extra Light is 2mm thick. Light is 2 and 1/2mm thick. Medium is 3mm thick. Heavy is 3 and 1/2mm thick. Super Heavy is 4mm thick. Extreme Heavy is 6mm thick! Please be sure to measure your fur friend for accurate sizing. Please use the second and third picture for sizing information. Additional Details ------------------------------ Product description: Pro Quality Strong Chain Collars for Dog Training and Walking Ultra strong, great quality Chain Collars feature rugged electronically welded steel construction, for the smart training solution that's built to last. High quality Steel. Thicknesses are as follows: Extra Light is 2mm thick. Light is 2 and 1/2mm thick. Medium is 3mm thick. Heavy is 3 and 1/2mm thick. Super Heavy is 4mm thick. Extreme Heavy is 6mm thick! Please be sure to measure your fur friend for accurate sizing. Please use the second and third picture for sizing information.