Extra Control Collar For Smart Dog 5 Acre System Extra collar receiver helps keep dogs safe in the yard For use with Smart Dog SD-2000 or SD-2050 Pet Fencing Systems Delivers electronic stimulation dogs quickly learn to avoid Battery-operated water-resistant receiver; collar fits necks 8 to 21 inches Receiver measures 2 by 1-1/4 by 1-1/2 inches; limited lifetime warranty