The rules of billiards aren't hard and fast; they vary depending on the venue and what game is being played. However, some pool game rules are pretty standard, so familiarity with these rules can help you get off on the right foot wherever you play.
Always Hit the Cue Ball
This may seem obvious to experienced billiard players, but novices may not realize that you can only hit the cue ball to move the other balls around. An inexperienced pool player might walk right up to a table and hit a random ball, but that isn't a legal play. You can only strike the cue ball with the tip of the pool cue.
Failure to Hit a Ball Is a Foul
When you shoot the cue ball and fail to hit the object ball or the bumpers, it's called a foul. A scratch is also considered a foul, as is hitting the wrong ball when playing a game that requires you to hit the object balls in sequence. When you foul, it becomes your opponent's turn to shoot.
If the cue ball follows an object ball into the pocket or ends up in a different pocket, it's called a scratch. If the cue ball leaves the table entirely, it's also a scratch. A scratch is a type of foul. Your turn is over, and it's your opponent's turn to shoot. In most sets of rules, a scratch forces you to put an already pocketed ball back on the table, leaving you at a disadvantage.
Ball in Hand
The concept of a ball in hand can be tricky. Technically, when a player fouls, the opponent gets the ball in hand. This means that the opponent can place the ball on the table in a position that's advantageous to him or her. The idea of a ball in hand is to eliminate intentional fouls.
When the ball is in hand, the player can only place it behind the head string; which is behind the second diamond from the from the end of the table, opposite the object ball, which must be located beyond the head string. The ball is considered in hand until the player hits the cue ball with the pool cue. That means that the player can move the ball again once it's placed on the table, until he puts the ball in play by striking it with the pool cue.
Called-Shot Games Have Specific Rules
If you're playing a called-shot game, the shooting player must call the shot before he or she strikes the cue ball with the pool cue. Calling a shot does not require detail, but the player must indicate which ball he or she intends to hit into which pocket.
While general rules can get very specific about fouls, scratches and the like, some common sense rules apply too. For example, you cannot take a shot while any balls are moving. You cannot touch the cue ball or any object ball prior to taking a shot. When you strike a cue ball, it's considered contact, even if you were taking a practice shot and didn't intend to strike the ball.
While general pool game rules apply to most billiards games, some variations include specific rules about the order in which you can hit balls, resulting in penalties for hitting the wrong ball or hitting a ball out of order. Most games also have pool table rules, such as where you can position the ball when your opponent scratches, or even that you must have one foot on the ground when you make a shot. When playing a variant, make sure you understand all the rules or you could find yourself losing.
Four-ball billiards is less common in the United States, but European and Asian versions are extremely popular. Learn about these variations and their scoring.
The history of billiards is a long, rich subject. The billiards game you play today has come a very long way from its origins as a lawn game in France.