Oops, I meant Louis XIV's reign. "In 1641 [Louis XIII] introduced a thaler-sized silver coin originally called a Louis d’argent and valued at 3 livres tournois. This value was associated with the stabilization of the écu d’or at that value in 1577, and the new silver coin came to be called an écu... From 1690 to 1725 rates were unstable and new écus were issued, and existing écus revalued. After 1726 the final écu remained stable at six livres tournois" from the wikipedia article on écu.