Now that you know all about the object balls, you need to learn the general rules that apply to the balls during play. Although there are specific rules for every game, there are some standard rules and terminology that will help you as you begin to learn. Once you have these general rules and terms in your memory, learning the specific games will be a breeze.
A legal shot is one that is generally permissible in most billiard games, while an illegal shot is a foul and you may be subject to penalties. Here is a list of some general rules:
Only the tip of the cue can touch the cue ball.
You can touch the cue ball only once per strike.
The cue ball must strike the correct object ball with the intention of sinking that object ball into a pocket. If you don't sink the object ball, then at least one ball on the table (including the cue ball) must touch a cushion.
That last rule might seem a little confusing, but what it means is that although the goal is to sink an object ball, to hold on to your turn, it's not absolutely necessary to sink the ball. For example, you hit the 3 ball into the 5 ball, which then hits the 6 ball into a cushion. You didn't pocket anything, but your shot is still legal because at least one of the object balls or the cue ball in your strike hit a cushion.
Some games require that you hit the lowest ball on the table first. This doesn't mean you have to pocket that ball. Take the game of nine ball for example: You have to strike the lowest ball first, but if you hit the 1 ball into the 2 ball, and pocket the 2 ball but not the 1 ball, that's still a legal shot. In eight ball (depending on the version you are playing), you may be required to strike one of your own balls first before striking the ball of your opponent. For example, if you are solid, you must first hit one of your own balls before striking your opponent's striped ball for the shot to be considered legal.
In the case of “call” shots, you are required to announce (or call) the ball you intend to pocket and the pocket in which you intend to sink that ball. If you don't pocket that ball or you pocket the ball into a pocket other than the one called, you forfeit your turn. When you pocket the wrong ball in a call-shot game, you will have to return that ball to the table by spotting it, which means setting it on the foot spot or on the vertical imaginary line down from the foot spot if another ball is blocking the foot spot.