Invisible Fence R21 Compatible Dog Fence Collar for Replacement or Addition to Existing Invisible Fence Brand Dog Fence System - with Backup Collar Strap Bundle INVISIBLE FENCE COMPATIBLE - These Collars Will Work with Invisible Fence Brand Inground Fences Prior to the 800 Series, Including ICT-700, ICT-725, ICT-750, ICT-775, ICT-150, ICT-250, ICT-100, ICT-75, ICT-50, AT-950 Indoor Transmitter, CT-4002 NOT COMPATIBLE - These Collars Will NOT Work with Invisible Fence 800 Series Systems or Later Fence (Including the Titanium Series). . Will NOT work with Boundaries Plus Fence. Nor Will They Work With the Shields Indoor Avoidance Transmitters FREQUENCY SELECTION - Most Invisible Fence Brand Underground Fence Systems Run on 7K, by Default. Systems that were Setup with Neighboring Fence Systems in Place May Run on 10K, to Avoid Interference AMERICAN MANUFACTURED - These Collars are Proudly Manufactured Right Here in the U.S.A. with a History of Dependability Behind Them Each Pet Fence Collar Order Comes with a Free eXtreme Dog Fence Backup Collar Strap