By the early 1800's, most large American cities had public gaslight until
electric lighting took its place in the later part of the century. Early electricity,
undependable and lighting fixtures that could use both gas and electric
became popular, although short-lived.
In general, whether a vintage fixture was "born" electric or
gas, is generally determined by which direction the shades face, either up or
down. Shades that face upwards were made to accommodate gas jets and are
generally larger, usually with 4" fitters. Shades which face downwards were made
for electric light bulbs and have smaller 21/4" fitters. Although there are exceptions,
this is a good rule of thumb to go by.