A perimeter fence is usually used to define the boundaries of a protected area. The mere presence of a fence, no matter how high it may be, sends a clear, concise message to the onlooker as to where he can and cannot go. Therefore, when an entry does take place, there's little doubt that the individual intended to break in. "Overall, fences serve a useful purpose by defining legal boundaries, deterring the general public, and eliminating interference from wanderers and lowly motivated intruders" (Encyclopedia of Security Management, Robert L. Barnard, Butterworth-Heinemann, Newton, Mass, pp. 229). These barriers are also installed to prevent or delay criminals' progress into a fenced-in area. High fences equipped with concertina wire or barbed wire, for example, have an even greater slowing effect because the burglar knows that he can be injured by crawling over it. An outside fence is the first line of defense to the structures contained within it. Drawing the line outside the perimeter walls of a building increases the amount of effort that a criminal has to expend. It also delays his entry, increasing the likelihood that he'll be either seen and identified or caught by law enforcement authorities.