Christmas lights are lights used for decoration around Christmas. The custom goes back to the use of candles to decorate the Christmas tree in upper-class homes in 18th-century Germany. Christmas trees displayed publicly and illuminated with electric lights became popular in the early 20th century. By the mid-20th century, it became customary to display strings of electric lights as along streets and on buildings Christmas decorations detached from the Christmas tree itself. In the United States, it became popular to outline private homes with such Christmas lights in tract housing beginning in the 1960s. By the late 20th century, the custom had also been adopted in non-western countries, notably in Japan.