What is "strong side" vs. "weak side" on defense in American football? I'm trying to get a basic understanding of this terminology but am getting tripped up.
In the "old days (up to about 50 years ago), teams had two "ends" at either end of the line (i.e., 'tight' to the line and therefore 'tight end(s)') to receive passes. Other pass receivers included two "halfbacks" and one "fullback" situated behind the quarterback.
Then teams decided that they only needed one halfback behind the quarterback, and moved the other halfback onto the line of scrimmage as a second, "wide" receiver, who was "split" several yards away from the end. This way, there would be three receivers running from the line of scrimmage instead of two.
The side with the third receiver was called the strong side, and the other side was the weak side. Defenses would compensate by moving an extra defender to the "strong" side.
Here is a related question about defensive measures.
As Tom Au stated, from a backer's perspective, the ”strong side” can simply be considered the side with the most receivers.
But in even more overly simplified rule-of-thumb terms, the “strong side” is often the side of the ball with the most distance to the sideline – space – the most open-field to cover ... where you generally want your “strongest” [most athletic] players (and generally the most receivers, per above.)
For example, in most high school and some college football, a defense may lineup strong or weak keyed on only that; regardless to how the offense is aligned – indeed before they even break the huddle. If the ball is dead center a linebacker or “captain” will make the decision and call it out.
Also: the term ”weak-side” (the oposite of whatever was determined to be the strong-side) can easily be confused with a quarterback's “blind-side”. A right-hand QB will drop back and plant off his right foot – thus the left side of the offensive scrimmage line will be the blind-side – regardless of where the ball is on the field. A pro defense will generally have their best pass rusher(s) attacking from the blind-side so the QB won't see them coming and more damage can be had (turnovers). But that may or may not equate with the “strong” end or “strong” formation, or ball location, in terms of the linemen.
An offense will purposefully confuse and pick apart a defense by overloading, and vise versa. Announcers will occasionally flip-flop these terms too, so it can get complex and thus confusing from a defensive perspective, as these things can change around just before the play starts.
The strong side is the side where the Tight End is lined up. Usually the right side, to help the Right Tackle block and provide a check-down option to the Quarterback.
Now, the (a bit) more long explanation: The Football rules require the Offense to field seven players at the line prior to the snap. The whole (modern) offensive line have 5 players: two Tackles, two Guards and a Center.
Usually, the last two players are the Tight End and the Split End receiver. The Split End (sometimes called the "X" receiver) aligns on the line, "covering" one of the Tackles.
The Tight End does the same, but instead of lining far from the OL bunch, it lines directly besides one of the Tackles-- usually the Right Tackle. This gives the line an extra blocker, one that is also an eligible receiver.
The need to support the run defense and cover the TE made, at some point, Defensive Coordinations, recruit almost linebacker-sized Safeties and moving them closer to the line. This, thus, became known as the "Strong Side"
And, as you probably guessed by now, "Strong Side" makes no sense in no-Tight End formations (4WRs 1RB sets, 5WRs, 3WRs 2RBs).
First, as a coach you will specifically coach your team on how to call your strong or the weak side. So just the call itself can vary greatly from team to team.
Second, most teams today either count eligibles or count bigs. We might consider the strong side of the formation the side with the most eligibles on it. We might also only count bigs. The term bigs can vary too. Sometimes bigs are TE/FB. Sometimes we throw the RB in with bigs.
Third, how we count for each team will depend on the teams tendencies, players, and formations. For some teams left might always be strong (because opposing team has right handed quarterback that boots out on half the passes). I have had coaches come over to me after games and compliment our team for bottling up their quarterback... Did you think if you throw out 3 receivers to the left that I was going to let your QB roll right all day?
Fourth, sometimes we don't even count. So I got a tiny DE that is lightning fast and a bigger DE that stuffs the run. Well I want my fast DE on the side of the QBs throwing hand (might be opposite in the NFL but in all other levels you want your speed on throwing hand). So when I have this type of arrangement I might have a complimentary OLB/safety combo. On the slower DE side I want a faster OLB. What I am saying is that sometimes strong is always left. No matter what because we want to force a team to match our players vs the other way around.
So the answer is - It is up to each coach. So if you face a team that goes single back, twins on each side in the middle of the field what is the strong side? Whatever coach tells you it should be. As a coach your players should never be surprised and should understand every scenario. On the flip side, it could just be strong left all day long.