The common rules

American Football is a very physical sport, but the game still has some rules. The most common rule that we can see on television is the offside or the false start. The rule is pretty basic: you can't cross the ball before it's put into play. If you do, the penalty will be of 5 yard against your team.

Another common call is the holding or illegal block. While in play, it's against the rules to block someone from behind or hold them when they pass you. If you do hold someone, the penalty is 10 yard against your team.

The last most common is pass interference. The penalty is given when a defensive corner back, or DB, touches the receiver before he catches the ball. If the defense makes the penalty, the ball is put where the penalty was made and it's first down automatically. If the offence is the one taking the penalty, its 15 yards against your team.

The points

American football has a very interesting score system. Every touchdown is worth 6 points, but the conversion kick adds the possibility to make an extra point. It's even possible to keep the offence on the field after a touchdown for two points. They have one down to make a touchdown at the 5 yard line.

You can make 3 points by doing a field-goal if the team succeed to kick the ball between the two post.

The composition of the teams

Every team is composed of tree units: offensive unit, defensive unit and special teams. First of all, let’s talk about the defensive team: the unit is composed of 12 players, defensive line-mens, corner back, backers and a safety. Here you can see a schema of a 4-3 formation which is composed of 4 defensive line-mens, 3 linebackers, two corner back and two safety player, this is the normal formation. 

Secondly, there is the offense unit which is basically built with five offensive line-mens, two running backs, four receivers and a quarterback. Here is a schema of a basic offense formation:

Finally, there is the special teams, these units are made with offensive and defensive players and are coming on the field on different moments in the game. The first one is the punt; the punt unit is coming on the field when the offense unit, is unable to reach the touchdown zone or when they are too far for a field goal, there is how it works, the team bring on a punter, to kick the ball as far as he can so the defensive players can have a good position on the field. Here is a schema of a standard punt unit (the punt unit is on blue):

 Next, there is the kickoff unit. This unit is specified in long range kick at the beginning of the game, after half-time and after each touchdowns. The goal of the unit is to kick the ball as far as possible so that the other team starts in offence deep in their zone. The ball is placed on a tee in the middle of the field. The unit is usually organized in a symmetrical way. The kicker is in the center and the remaining players go down the field as fast as they can to tackle the player with the ball. Here is a schema of a standard kickoff formation:

The last special unit is called field-goal. It's job is to kick the ball between the two post at the opposing team's side of the field. Field-goal is used to convert a touchdown and add one more point to the score. The second reason why the field-goal team would be on the field is to kick the ball between the post on a fourth down to make three points. In contrary of the kickoff, someone holds the ball during the field-goal. When the ball is snapped, the «holder» takes the ball and puts it on the ground so that the kicker can kick it.  Here is an example of a field-goal:

The Downs

The downs, this system only use in American football permit to the game to be a lot more interesting. This is how it works: the offensive unit has to go to the touchdown of the other team. For this, they have four downs, but they are restarting the countdown every time they achieve to go over 10 yards. Here is an example: The offense starts at the 10 yards line at the left, they do 2 downs and they achieve to run 20 yards (see point 1 for the yard system), they start at the 30 yard line and they restart the down to first. The goal for them is too run into the endzone on the other side. Let's say that they did not achieve to run 10 yards in 4 downs, normally if they can't get first down after 3 plays, they will bring a kicker on the field so that they can clear their side of the field to give a good field position to the defense.  

The Field

An NFL (National football league) field is the classic field in terms of dimensions but there's also the CFL's (Canadian football league) size. First of all, the NFL field: the field is 100 yards long and is 53.3 yards large, the endzone is barely 10 yards long and 53.3 yards large. Second, the CFL's dimensions, the field is 150 yards long and is 65 yards large, the endzones are 20 yards long and 65 yards large. Of course, the size of the field will have a great impact on the type of play the teams will adopt. When the field is large, it's easier for the teams to run the ball. On the other side, if the field is not as large, passing the ball will be easier.

The size of a field in the NFL

The size of an field in the CFL