Slide rules were sometimes distinctive by the materials from which they were made. Early rules were often made from boxwood and other woods. By the late 19th century, German manufacturers and Keuffel & Esser of New York City had not only switched to the more uniform and durable mahogany but were also coating the wood with early forms of plastic (celluloid). Around the turn of the 20th century, Japanese firms used bamboo, which did not expand and shrink as much as wood, thus reducing errors in the results of calculations. Later, Pickett slide rules were notable for their aluminum construction and proprietary yellow color. Although the rules tended to be less affordable and popular than wooden rules, manufacturers have used brass and other metals throughout the history of slide rules. Plastic and paper became increasingly widespread for inexpensive rules in the 20th century.