The BT-502A comes with one transmitter and two collars color-keyed (Red and Blue) to the transmitter buttons. The collar receivers feature a low-battery indicator and internal antennas. The new reflective Red and Blue collars are adjustable to fit almost any dog. With the color coordinated collars and separate control buttons, you will never correct the wrong dog again.This economical and effective training system is not only the choice of professionals with one or two dogs, it is very popular as an at-home trainer. The major difference between this system and other systems is that it is designed for up to two dogs. So this compact remote trainer is ideal for reinforcing commands and stopping unwanted behavior such as digging, chewing and barking - from up to 150 yards. It is also a great trainer and allows you to take both dogs on walks in the neighborhood, trips, and on vacations.