What are fences in bowling?

Before computerized score keeping was introduced to Bowling, all scores were recorded and tallied by hand. Often when a bowler or team was not scoring well, the scorekeeper would draw a dark black line along the right edge of the frame. This was called a "fence" and would indicate that they poor performance was blocked and that improvement would occur. It was, of course, just a superstition, but it served as a mental rallying point.