Chain size: 2.5mm x 55cm, Note: This training collar is a bit large and heavy, suitable for puppies. Made of stainless steel plated steel, durable, rust and rust-proof, limited closure ensures dog safety, ideal for pet training. Designed for showing or obedience training or walking, there are no sharp points or rough edges that can scratch your dog's skin or snag their fur when used. FOR Escape Artists: A popular solution for dogs who can escape from a traditional collar; the Martingale design keeps your dog securely on a leash so you can enjoy your daily walks and adventure. Martingale Style Limited Collar: The choke collar is designed to make your dog's training safe and effective. Traditional choke collars are aggressive and use uncontrolled force to correct negative behavior. Martingale-style collars tighten under tension, but only to the extent of giving your dog a (subtle) controlled training cue. We specialize in pet products, and all our materials are carefully selected and strictly inspected during the production process. If you have any questions about the product, please feel free to and we will help you within 24 hours. Note: Due to the light and screen setting difference, the item's color may be slightly different from the pictures. Please allow slight dimension difference due to different manual measurement.