In this series of videos Prof. Patterson develops a web application version of Reversi (aka Othello) using HTML, CSS, Javascript and jQuery. The infrastructure is managed by Github and Heroku. By watching these videos and following along on a computer, you should be able to build the same application on your own. This is the fourth in a sub-sequence of tutorials in the series that focuses on enforcing the rules of the game. This video shows how to get the client to communicate to the player which moves are legal, so that they player can't submit illegal moves. This video is used in courses taught by Prof. Donald J Patterson (@djp3) at UC Irvine and Westmont College.