Defending spread passing teams often requires a numerical advantage. The Triangle concept is a good way to get that advantage and can be effective versus 2×2, trips, and empty formations.
In the Triangle concept, we will always have one deep defender (D), one outside defender (O) and one inside defender (I). The O keys the two receivers from the outside-in. He will cover the first outside release man to man. If both receivers go vertical, the O will cover the outside most receiver from the outside in using low shoulder position since he will have deep help. The I is a mirror of the O. The I keys the two receivers from the inside-out. He will cover the first inside release man to man. If both receivers go vertical, then the I will cover the inside most receiver from the inside using a low shoulder position since he will have deep help. The D keys both receivers. If both release vertical, he gains depth and stays over the top of both receivers as the Topper. If both receivers go out, he takes the second out route man to man. If both receivers go in, he takes the second in route man to man. If one receiver breaks his route off, either inside or outside, the D will take the remaining receiver man to man.