Below is a diagram of the standard defensive alignment techniques and gaps for football.
Defensive gaps refer to the space between the adjacent offensive lineman. Defensive alignments for the defensive fronts are identified by the in, over or outside shading of the defensive lineman to the offensive lineman. The diagram above refers to one of the best standards of naming defensive front alignments.
In this defensive technique numbering system, even numbers refer to headup & inside techniques and odd numbers for outside techniques, except for 7. The 7 technique is the only inside technique identified by an odd number. The letter “i” is used to specify an inside technique, 2i, 4i. To identify gap alignment you add gap at the end, 3 gap, 4 gap or 7 gap, and the center guard gap is a 1 gap technique.
Source: Coaching Offensive Lineman by Dave Christensen and James A. Peterson
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