Cover-3 — Three Deep, Four Underneath

Cover-3 puts the two corners and the free safety in deep thirds and four underneath defenders in zones (curl/flat, hook, hook, curl/flat). One safety drops down to be the eighth man in the box (the 'sky' rotation) or the corner squats while a safety rolls (the 'cloud' rotation). 'Buzz' is when the safety drops to a hook zone and a linebacker becomes the deep third defender.

It's the most-played defense in the NFL because it lets you play 8-in-the-box without a free runner downfield. The trade-off: cover-3 is soft outside the numbers (the corners are 12+ yards off) and the seams are stressed when both safeties bail.

Four verticals attacks cover-3 by stretching the deep middle. The seam routes, run between the curl/flat defender and the deep safety, win against single-high. Crossing routes (drive, dig) eat the underneath zones because the linebackers can't cover both hooks and react to a deep crosser. Modern offenses tag the seam read into RPOs to make cover-3 pay for staying single-high.