The purpose of the line is to make both teams look equally attractive to the gambling public. Perhaps no one would take Navy as a pick-em, for instance, but they might look attractive with 28 extra points. The opening line is set by bookmakers who try to gauge the public's opinion of the game. They want to balance their book, or in other words, make sure that they receive- an equal amount of money on both sides of the line. The reason for this is simple -- bookmakers don't like to gamble! If their book is balanced, they can't lose since they get to keep about 4.5% of the total amount of money wagered. If their book isn't balanced, they're gambling just like the public, and there is a good chance they could lose.

If too much money is wagered on one side of the line, the bookmaker moves the line to encourage gamblers to take the other side. For example, if too few gamblers take Navy at +28, the line might move to +29 to make Navy look more attractive. Money moves the line. If the line is moving further and further away from Navy, you’ll know that most of the money is being wagered on Notre Dame. During the week before a game, the line might constantly move. For example, it may go from 28 to 29, back to 28.5, and so on. However, the opening line seldom differs from the closing line by more than 2 or 3 points. Bookmakers try to avoid moving the line too far in the same direction. If they do, they may be middled, which means they'll have to pay out money on both sides of the line. For example, lets look at a hypothetical situation where the line on the Notre Dame-Navy game opens at Notre Dame (-27). The majority of the gambling public feels that Notre Dame will win by more than 27 points, so nearly all of the money is being wagered on Notre Dame. To entice gamblers to put money on Navy, the line keeps moving away from Navy. It closes at Notre Dame (-32), meaning that the line moved by a full 5 points.

Notre Dame winds up winning by a score of 40-10. The book has to pay all the gamblers who took Notre Dame and laid from 27 to 29.5 points earlier in the week. It also has to pay the gamblers who took Navy plus 30.5 to 32 points later in the week. Some gamblers may have wagered on Notre Dame early in the week and Navy later in the week and won both wagers. This is what it means to be middled. Normally a sportsbook only has to pay the people on one side of the line. In this case, they have to pay out money on both sides of the line. To avoid being middled, sportsbooks seldom move the line by more than 2 or 3 points in the same direction.

In rare cases, large sums of money will unexpectedly show up on one side of the line. The oddsmakers have set a line that (in their opinion) makes each team look equally attractive to the gambling public. Even so, nearly all of the money is being wagered on the same team. This is called unnatural money. When a sportsbook feels this is happening, they may elect to investigate and see where the money is coming from. In the meantime, they may take the game off the board and refuse to accept further wagers. In other cases, injuries to key players or an unusual situation surrounding a team will force a game to go temporarily off the board.

As it turns out, the Las Vegas oddsmakers have years of experience setting lines, and have developed an excellent feel for how the public will wager. To most people the line seems like it's about right. About half the gamblers will like one team, and about half will like the other team. The money will usually start to fall fairly evenly on both sides of the line, which is exactly what the oddsmakers want. If they do a good job of gauging the public opinion of a game and set a good line, everything else takes care of itself. Not much line movement is necessary, the book stays balanced, and they collect about 4.5% of the action. Although sports gambling has more favorable odds than almost all other forms of gambling, the odds are still against the player.

Knowing that the odds are against you, can you still beat the line? Sure. Many gamblers routinely beat the line. The key is learning as much as you can about the relative strengths and weaknesses of the two competing teams. Respect the public’s perception of a game, but don’t assume that it’s always right. Do your homework and form your own opinions. The single most important thing to remember about handicapping is:

The line is based on the public's perception of the game, and not on the relative strength of the two teams.