We didn’t have a fence around our backyard when I was a kid. On days with nice weather we would let the dog hang out back there. His collar was attached to a line that went to a metal loop in the middle of the yard. This gave him about a 100′ wide circle to romp around in. He understood where this invisible circle was. If you were out playing fetch with him he would stop at the edge when you threw the ball too far. But for some reason, squirrels gave him temporary amnesia. If a squirrel ran through his play area he would chase after it full speed right to the limit of his leash which would then flip him onto his back as the squirrel scampered up a tree and out of sight.