From a reader: "What's the purpose of the 6-7 men on the line of scrimmage rule?  What exactly is the rule?  Often the linemen are lined up slightly off the ball, but they're considered on the LOS."
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The basis for the seven-man rule (it is always seven men on the line of scrimmage for the offense and field goal/PAT/punting units, in all circumstances, never six or eight men) is so ingrained in modern strategy that it's barely mentioned, even though it's at the heart of every system, formation and non-trick play devised for the last hundred years. A lot of fans probably don't understand what makes an "illegal formation" illegal, or why it's illegal to begin with.

The reason: death. Or something like that, according to the turn-of-the-century prohibitionists who attacked football in the same vein as poverty and alcohol. Most teams by 1889 (the year after the rugby-influenced rule restricting any teammate from running in front of the ballcarrier was rescinded) used some variation of the slow-moving "V wedge" on kickoffs, which functioned something like a World War I trench attack looking for a breakthrough in opposing defensive lines. Kick returns still employ the wedge concept, but the really dangerous innovation was Harvard coach Lorin E. Deland's "fling wedge," described thusly in a game against Yale in 1892: The flying wedge and other "mass" formations were specifically outlawed in 1894 by restrincting the number of men who could go in motion before the snap to three, but under the new scrimmage system, coaches used the same concept to pull their linemen into the backfield to allow a running start, send backs in motion before the snap, and still achieve momentum toward the line. The most effective way for the defense to "bust the wedge" of oncoming blockers was the bloody business of hurling itself at their knees. The subsequent carnage (and, it should be noted, the regular cheap shots and fist fights accompanying it, which had nothing to do with rules and everything to do with pure, seething hate) was enough to force Teddy Roosevelt to threaten to outlaw the entire sport in 1905, with the result of introducing helmets, knee pads, the forward pass and, in 1910, a rule mandating seven men on the line of scrimmage and restricting motion to keep the offense from gaining momentum toward the line.

This restriction more than any other is the main reason offensive philosophy is more cyclical than evolutionary, because the five-man interior line is a staple almost by definition - if you have to have five guys on the field who are not eligible for a pass and have to line up along the line of scrimmage, they may as well be the most giant monsters you can get your hands on to push the other team around - and there are only so many options with the rest of the guys on the field. That's why your standard formation for years has looked like this:

You can mix and match the eligible players (both WRs, the TE and the running backs) and move them anywhere you like, as long as two of them are somewhere along the line of scrimmage and somebody is available to take the snap, but even when you spread the field...

...it's just moving a couple of pieces on the board. It's worthless to try to stretch the ineligible linemen across the field when the only function available to them, by rule, is blocking. Because of the seven-man restriction, there is a very limited set of alignment possibilities among the five eligible receivers, which allows defenses to make some automatic assumptions and makes the eligible/ineligible distinction pretty easy to spot when those possibilities become familiar.

Those assumptions can be (though very rarely are) exploited. LSU, for example, took advantage of the quick calculations defenders make in their alignment in the BCS championship game, when they came out near the Ohio State goalline in a weird formation that looked something like this:

A defender looking at that formation would reflexively discount the "TE" at the end of the line, because that spot is always occupied by an ineligible offensive lineman. But LSU snuck tight end Richard Dickson into the left tackle spot (he is eligible for a pass because the WR to that side of the formation is not on the line; that receiver is "replaced" on the line by the LT who lined up split wide to the opposite side of the formation, but who remains ineligible for a pass because he is covered by another WR) and hurried the snap before Ohio State could react to who was where. The result:

It's the kind of thing that only works once, though.

As far as penalties, illegal formation flags are almost always the wide receiver's fault: officials announce the number of the last lineman on the line, but the receiver who didn't establish himself as the seventh man is the culprit. As far as linemen not lining up along the line, sometimes a tackle anxious to get into pass protection is hit with a flag if he's too far behind the line, but it's so common as passing increases that officials let it slide unless the guy's egregiously in the backfield. It's a judgment call, and frustratingly ticky-tack when whistled.

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