DESC: These extra collar receivers are compatible with Innotek's Ultra Smart In-Ground Pet Fencing Systems; IUC-4100, IUC-4200, and IUC-5200. Add additional collars to your yard's pet containment system to contain additional dogs within your pre-set perimeter. ,, Innotek Ultra Smart In-Ground Pet Fencing Systems are sold separately and can be found through item search at item numbers; #38-884-679-00 (IUC-4100) #38-884-779-00 (IUC-4200) #38-884-781-00 (IUC-5200) ,, ,, Features: ,, ,, Ready Test- ensures correct collar fit, graduated battery status, and warns of potential system problems for greatest pet safety ,, Contoured, integrated collar-30% thinner than other collar designs, a fraction of the weight, more comfortable on your dog ,, Waterproof-collar and transmitter stand up to rough punishment, even submerged underwater ,, Rechargeable collar-fast-charging, long life lithium ion batteries ,, Expandable-add multiple dogs to the in-ground fencing system with extra collars (sold separately)