Insulators of glass or porcelain have been made for use on telegraph, telephone, or electric poles since 1844. Billions of insulators in hundreds of styles have been made. The most common glass insulators are clear or aqua; the most common porcelain ones are chocolate brown. The most desirable are the threadless types made from about 1850 to 1870. Collectors identify insulators by their shape. Glass insulators are identified by their CD (Consolidated Design) numbers and porcelain insulators by their U (Unipart) numbers. Information about these numbering systems can be found online. For more information, explore our identification guides for glass, bottles and porcelain.