The Telleruhr or dish clock was popular in south Germany and Austria in the late 17th century, and the idea was revived in the 20th using a china plate instead of a metal one. The movements were similar to the going part of an alarm clock with an open mainspring and were controlled by a balance, but the mainspring arbour had a winding square allowing the clock to be wound through a hole in the dial. These clocks were mass-produced by the Black Forest factories in the 1920s, and were, in effect, a development of the china dials found on Black Forest wall clocks of the mid 19th century. Their fragility has greatly reduced their numbers and they are now seldom seen.