By the Book

"What you must learn is that these rules are no different than the rules of a computer system. Some of them can be bent. Others can be broken."
-- Morpheus, The Matrix

Rules in games define the boundaries to which game play is integral to. Without rules, games can become a jumbled mess of ideas and information that have no connection and no underlying purpose for their existence. Rules are necessary as placing constriction on game play elements as to what a player can and can't do tie directly into how the game itself works.

Rules are not necessarily just guidelines that a player should abide by when playing a game. The collision detection a player faces with the floor, wall and ceilings are a simple example of rules which define the boundaries of game play. Not being able to shoot your team mates, build where they have built or take their stuff are examples of restrictions a player has over their friends. An example of these sort of rules becoming a game play element can be clearly seen in (and I hate to use this example yet again, but there is simply nothing out thats better) Team Fortress 2.

In TF2, every player has a hit box and a collision box that prevents them from passing through things or taking damage incorrectly. However, while it is not possible to move through the opposition's collision box (i.e. run into each other), you are able to pass through friendlies and certain friendly fire with no noticeable collision restrictions (though you are pushed out of them if you attempt to remain within them). This adds a simple yet effective gameplay element for uncovering the 'Spy' class in the game. The Spy, who has the ability to make himself appear like the enemy, is not granted the same collision detection as the players he is fooling. This therefore makes the detection of Spy's as peaceful as running into your team mates, purposefully. Anyone you do not simply pass through would therefore quite likely be a spy, therefore the delivery of airborne lead or explosives should be placed towards the location of the spy quite promptly. This tactic is usually employed by suspicious team mates who are either conservative on their ammo or are brandishing some form of deadly melee weapon.

Below: TF2 - Irritating spy is irritating...


Rocket jumping, found in many action based FPS's is another example of how a rule can be used to change how a game is played. Most explosives in games are programmed to have some form of area of effect knockback or pushback to entities in close proximity to it. This usually is represented on the positive Z axis as well. It is therefore not surprising that many players often use this extra 'boost' from explosive weapons/objects to reach extraordinary heights while jumping.

Below: TF2 - Awesome rocket jump is awesome!

Demonstration: How to Rocket Jump

These sort of game play defining rules are not only present in computer/console games. Even in a card game like snap, a physical pain and endurance element can be added depending on how quick your reactions are and how hard the slower people hit your victorious hand.

From a games design perspective it is important to realise how the various rules of a game will change the game play, and to either cater or cut down on certan things if the results are not desirable (e.g. exploits, unfair game play). This is where extensive testing from both closed and beta levels would be necessary to balance said game play elements derived from the 'influence' of rules.