On the D&SNG K-28s and K-36s, and the C&TS K-36s and K-37s, this particular valve motion is called the Walschaerts after its Belgian inventor, Egide Walschaerts, who patented the mechanism in 1844. Although not the most efficient valve motion, the Walschaerts was much more durable and required considerably less maintenance than other types—leading to wide-spread adoption in the United States, despite its late introduction.