I am lucky enough to work on the Yellow Rose (contemporary western) line for The Wild Rose Press. Cowboys, cowboy, and more cowboys....ah yes, such a sacrifice LOL. And there is one set of words that more often than not get mixed up in every three out of four manuscripts I edit...
Rain: moisture drops from the sky. (To be honest, this is the one version that rarely gets mixed up, but as it is part of the homonym set, I felt it needed to be included overall)
Reign: to rule over something such as a country.
Rein: the long straps attached to a horse's bit in order to help guide the animal, or to check or guide (as in pulling on the reins to stop or turn direction).
Now, as for the last two, these get mixed up all the time because a cowboy doesn't reign in his horse, he reins in his horse. A king doesn't rein over his kingdom, he reigns over his kingdom. Amazing how one letter "g" can change the whole meaning.
But, here is the tricky part when using an abstract description. He doesn't try to reign in his emotions, but instead, he reins in his emotions. An easy way to remember is thinking of it as the character is holding back/controlling all his pent up emotions, kind of like a cowboy controlling the reins of a feisty horse.
Reign is to rule--think of the "g" as if to govern over. Rein, on the other hand, is a strap or to hold back--no "g" needed.
Just for fun, let's put them all together...
Storm clouds reigned over the darkening sky as rain poured down on the cowboy attempting to rein in his fear-ridden horse.
Do a Find in your manuscript for reign and rein and make sure you are using the right word spelling.