Throughout the 20th century, the trend in textiles has been toward lighter-weight materials. Better transportation and improved indoor heating have made warmth a less important aspect of clothing for most people than it was in earlier centuries. Since 1900 the weight of average clothing fabric has fallen by more than one-third. When people need extremely warm clothes, they tend to wear special outer garments such as parkas that today are typically made of lightweight synthetic materials.
Synthetic materials, produced from chemical compounds rather than from plant fibers or animal hair, have provided less-expensive substitutes for natural fabrics. Synthetic materials can also be superior to natural fibers in strength and durability. Rayon was an early synthetic substitute for silk. Nylon, a synthetic fabric introduced in the 1930s, was another early substitute for silk and quickly became the fiber of choice for women's stockings. Polyester, a form of plastic, was introduced in clothing in the early 1950s. Blended with rayon or cotton, polyester found its first use in so-called wash-and-wear fabrics that needed little or no ironing.
Synthetic fibers fell out of favor in the late 1960s and 1970s, but new kinds of polyester that are more durable and have a softer, more natural feel to them have become increasingly popular in the late 20th century. Synthetic fibers such as spandex have revolutionized clothing by making possible the production of extremely flexible, form-fitting garments. Other synthetic fibers, created for special purposes, range from lightweight but extremely warm or water-resistant fabrics, such as polypropylene and the composite polymer Gore-tex, to woven, bullet-proof fabrics such as Kevlar that serve as body armor.