Titan® Extra Links for Dog Prong Training Collars may be added or removed for proper fit. Available in packs of 3 in fine 2.0 mm, medium 3.0 mm, heavy 3.3 mm, and x-heavy 4.0 mm. Titan chain is argon welded and chrome plated for maximum strength and durability. It will not tarnish, rust or break. Chain training collars are safe, effective training tools with proper use. They are not to be used for tie out.