When you are examining a football game that you are considering making a bet on, you will often find yourself faced with different betting lines based on the time of the week you are looking at the point spread or the Web site you are looking at. The lines are, barring injuries, set a week in advance. But most lines will see some movement throughout the week. Some games won't move at all, while other games can move by as much as a touchdown or more, especially in college ball.

In the simplest sense, the line moves because bookmakers want to have the same amount of money bet on both teams in a game. Oddsmakers want to set the lines so that the games are essentially a coin flip - bettors can make a strong case for both teams. If one team is getting more action than the other, the line will be moved to make the less popular team seem more attractive to the bettor. If a bookmaker has the same amount of money bet on both sides of a game then they have no risk and they are guaranteed to make money no matter what the outcome of the game. This is because of the vigorish, also called the vig or the juice. When you bet on a point spread game you have to bet $110 to win $100 (you can often find lower vig rates online if you look around). If a game is perfectly balanced then the bookmaker makes $10 for every $220 bet regardless of the outcome.

In reality, there are dozens of reasons that a line can move. The line is set by someone like the Las Vegas Sports Consultants, the company that provides lines for the majority of Nevada sportsbooks. Most Las Vegas books use those odds as a guideline then adjust them according to their customer's betting patterns. The first reason a book may change their line is if their experts have a strong opinion on the game which differs from the original line. If the line was set to make one team a favorite, but the bookmakers feel that the other team should be favored, they could move their line to try to avoid a disaster caused by heavy action on one side.

The line may also be moved because of geographical location or other factors. Say, for example, that most of the bettors that use a particular bookmaker are based in Southern California. That bookmaker knows that they would see heavy action on USC regardless of the team they are playing or what the point spread is. They may move their line so that USC has to win by more points to cover in order to give themselves some more protection and to decrease the chances that they will have to pay out much more than they brought in on a game. This is referred to as "shading the line" in the sports betting industry.

Certain factors that occur during the week can cause a line to change. If the star quarterback for a college team is hurt in practice and has to be replaced by an untested freshman, for example, then the line will move to compensate for that injury and the decreased likelihood of a win or a cover by the team with the injury woes. If the bookmakers didn't move the line then they would get too much action on the team without the injury and they could stand to lose a lot of money.

The line will be moved at any time in the week if a disproportionate amount of money is bet on one team. A "steam move" is the name for a line move that happens as a result of a large amount of money being bet in a short period of time. Early in the week, a bookmaker can see 'smart money' bet on one side, and that can cause a steam move. That smart money can come from so-called 'wise guys' - experts that may have inside information or a strong opinion - or it can come from syndicates of bettors who combine their resources to make large bets. There is a school of thought that suggests that you should watch for these steam moves and bet on the team that is getting the smart money action. The problem is that you may already be too late if the line has already moved. If a spread started at 2.5, but was steam moved to 3.5, for example, then you could be too late. One in six NFL games is decided by 3 points (3 is called a key number in football betting because it is such an important factor in making betting decisions), so the smart money in this example would win while you would lose if you jumped in at 3.5.

The line can also move because of public money, though this will usually happen closer to game time, because that is when the public usually bets. As a general rule the public likes to bet on the favorites, so a line will most often be moved to make the favorite less attractive as a result of public betting. The line is usually slower to adjust to public money than it is to smart money. This is in part because public money can be more spread out over different books and different teams and may not show as sharp of an opinion, and in part because the public opinion generally isn't respected by bookmakers as the smart money opinion is.

Theoretically bookmakers work independently and can move their lines as they wish. In practice, though, line changes will happen at most books fairly quickly. The Don Best Odds Service is a real time service used by most sportsbooks that keeps track of lines and lets bookmakers know as soon as someone moves a line. If a few books move a line in a short period of time, like a few minutes, then most books will likely quickly follow unless they have a specific reason of their own not to. If the bookmakers did not move their line quickly they would risk getting a large bet because one team is more attractive at their line than it is in the rest of the market. That could be a very costly situation.