The traditional upholstery that is still associated with the word today developed in the early 17th century and was primarily found in the homes of wealthy individuals. Springs were first incorporated as part of upholstery in the 18th century and were helical in the 19th century, but today, they are a flatter shape. The 20th century saw upholstery modernized by new products and inventions like plywood, synthetic fibers, and liquid retardants.