I wanted to be able to let my cats out into our fenced back yard without worrying about them jumping over the fence or other cats getting in. My wife saw a system for sale on the Internet but I made my own for 1/4 of the price. Here's how I did it.
Step 1: The Theory Behind the Fence
In theory, cats will not climb a reverse overhang or jump out and over something when they can see straight down. I found the first part to be true so far, but my neighbor's cat definitely disproved the second part. The unwanted visitor managed to get in my yard, but he could not get out until I let him out and has not been back since.
By attaching plastic mesh garden fencing to my wooden fence, I have been able to keep my cats in and the neighbor cats out (for the most part).