This popular PetSafe obedience and yard-training collar has several new features . An on/off button on the receiver that conserves battery power without magnets. A convenient thumb wheel makes it quick and easy to change stimulation levels even when wearing gloves. Tone button feature can be used for silent praise or in conjunction with the stimulation button as a warning tone. The compact receiver makes it a great choice for training larger dogs. Note: The image shows the transmitter that is standard for this unit, but it also shows both collars, for large and regular dogs (#SD15). This unit will come with one collar, represented by the bottom image, to fit a larger sized dog. This is a one dog unit.