A marker is a sound that means the food is coming. It is not a command and it does not make the dog do anything. What it does is tell the dog exactly which half-second earned the reward, which is the difference between a dog who learns in three sessions and one who is still guessing after three weeks.
Plastic clicker and whistle in one unit on an elastic wristband, so it is on your hand rather than lost in a pocket. Eleven colours.
Clicker or whistle
The clicker is for close work: loose leash walking, sit, place, anything within a few metres. It is sharp, identical every time, and carries no tone of voice.
The whistle carries much further and is the better marker for recall on a long line, where a click will not reach. It also sounds the same whether you are calm or annoyed, which your voice does not.
Charging it
Before it means anything, click and immediately feed. Ten or fifteen times, no asking for behaviour, just click then food. Once the dog looks for the food on hearing the click, the marker is loaded and you can start using it to mark the moment you want.
Part of the Leash Pulling Kit and the Recall Kit.