A two gap style in football is a defensive pass rush technique in which defensive linemen attempt to plug two or more lanes between offensive linemen to prevent successful running plays up the middle. Each lane is the space in between each offensive linemen. A good rule of thumb for how many gaps there are is to count the total number of blockers and then add 1. For example, an offensive line with 6 offensive linemen and one fullback has 8 gaps (see below).
Similarly to the two gap style, one gap style is a pass rush technique used by the defense. However, one gap style has defenders only focus on only one of the gaps created by the offensive line. Defenders can get off to a quick jump as they know where they need to cover without reading the running back's movement, as is the case in a two gap style.
One gap style also places more defenders in what is known as 'the box'. The box is an imaginary area that spans along the line of scrimmage and extends five yards down field. Defenders who are in the box are focused on stopping the run. A one gap style can have up to eight defenders in the box, while a two gap strategy can have as little as five.
When watching how a defense handles the offensive team's running game keep an eye on how the linebackers handle their two gaps. Not only do they need to cover two gaps, they have to run to the line of scrimmage first in order to do so. By then, the play might already be developed and the running back may have already gotten through the hole. A linebacker needs to make a split second decision in this type of defense, so keep an eye on them if you want to know whether or not the play will be successful.
The rationale for using a two gap strategy is that it allows for the defense to use less total players in the running game and have more in the secondary. With each defender taking up two gaps theoretically a defense could stop the run with only five defenders as opposed to the usual seven or eight. By placing more of the defenders in the secondary, the offense has less options and less of a chance of succeeding in the passing game. This is especially beneficial against a team with a strong passing game.