Rein means to keep under control, to restrain. Reign means to hold royal office or to rule (as king or queen). These homonyms, due to their similarity in meaning, are often rendered incorrectly. Most idioms pertaining to control (free rein, rein in, take the reins) originate in horseback riding and should be spelled as shown. Idioms using reign rely more on governmental power and appear less frequently in colloquial situations (reign of terror, reign supreme, reign over).