The Law of Large Numbers dictates, given a large enough sample, a given probability will settle down into its distribution. Flip a coin ten times and it would not be surprising to get seven heads and three tails. Flip a coin a million times, and the distribution will level toward 50/50.
Well, brothers and sisters, it appears we’re due for a goddamn deluge of the correct usage of “rein” vs “reign” if these numbers are going to even out.
Rein: A long, narrow strap attached at one end to a horse’s bit, e.g., The sheriff tugged the reins abruptly to avoid trampling Gabby, the Town Pervert. Metaphorically, to curb or control something, e.g., The NBA asked the power forward to rein in his domestic violence behavior, or risk a half-quarter suspension.
Reign: The period during which a sovereign rules, e.g., The reign of Louis XIV. To hold royal office or to rule, e.g., Proving that no one ever went broke underestimating the lowest common denominator, The Big Bang Theory reigned its time slot.
The only mnemonic I can offer is to focus on the letter “g” in “reign.” Think “regal” or, for fellow pedants, Ultima Ratio Regum.