Glass insulators came out in response to the invention of the telegraph, as they insulated the wooden poles from absorbing the electric signals carried by the wires. Before the 1870s, most insulators were without an internal thread-- meaning they just sat on pegs unsecured. By the mid 1870s, threaded insulators came to be and have been made as such ever since. The most collectible insulators are from their first 50 years, aprox. 1850-1900 period, though many highly collected insulators were made into the American Great Depression. By WW2, most insulators became plain and their collectibility plummets, though I suspect European insulators will become popular as time goes on, as many crude and colourful examples were made into the 1970s. Currently, it is American, Australian, and British insulators that are most collected, with the Americans leading the way. Generally, most American insulators were made by Hemingray Glass Co., with 1871 and 1893 patents (mould and later drip-points.) In some cases, you see 1893 embossed on insulators that didn't debut until the 1930s, as the dates are only patents. Insulators embossed Brookfield or B were made prior to Brookfield's shut-down in 1921. These are the two most commonly seen glass insulators. Glass insulators are ascribed by collectors, not the glass-houses, a CD number for their style. U-numbers are given to porcelain insulators, but not to wiring insulators. Colour and condition make all the difference for desireability in insulators, with aqua and clear being the most common. But beware, a large number of modified or reproduction (often using original moulds) coloured insulators have flooded the market since the 1970s and are considered fakes by serious collectors.