The two prototypes of zippers came from Americans Elias Howe (U.S. patent in 1851) and Whitcomb Judson (Whitcomb Judson, patented in 1891 and 1893), which are called "self-locking buckles". Both use hooks and loops for twisting; the former is not commercially manufactured and the latter is used to tie shoes and boots. The modern zipper was invented in 1914 by Gideon Sundback, a Swedish-born Canadian who was an employee of the company founded by Judson. He replaced the hook and loop structure with convex and concave twisting, and applied for an independent patent in 1917, called "Separable Fastener". Zippers became popular on children's and men's clothing in the 1920s and 1930s. At this time, the two zippers are still fixed together at the lower end, and will be separated later to be used on the jacket.