The EDGE RT comes complete as a 1-dog system, but is fully expandable to a 3-dog system with the purchase of optional receivers (sold separately). With 1-mile range, and a fully waterproof receiver and transmitter, this system offers all of the features needed for most conditions. The 2-hour rapid charge Lithium Polymer batteries are housed in a separate compartment on the transmitter/receiver for easy replacement. The EdgeRT offers a selector/step dial with 8 "click-able" levels of stimulation. Nick, constant, and vibrating page options allow even more training flexibility. (EdgeRT comes with 1 handheld transmitter and 1 receiver collar with a black strap **additional receivers with different strap colors are sold separately)