By the outbreak of World War I in 1914, designers and manufacturers in the United States were creating functional apparel for women as they replaced men working in factories. Frivolity was replaced by a new emphasis on masculinity, consistent with the burgeoning suffrage movement that ultimately led to women's right to vote in 1920. Due to rising labor costs, designers began to simplify clothing construction.
In Paris, designer trendsetters including Gabrielle "Coco" Channel, Paul Poiret, Jeanne Lanvin, and Jean Patou further liberated women by allowing them to express themselves via fashion. The cultural and artistic atmosphere of Paris in the early 1900s created a breeding ground for change centered on youth and simplicity. An interest in sports and leisure-time activities fostered new markets for the fashion industry, as did the growing travel trends aided by the inventions of the automobile and airplane. Chanel was a major force in the creation of the "modern woman" through her innovative style that first graced the fashion scene in the 1920s.
By that decade, department stores were selling "ready-to-wear" clothing in dedicated "departments" featuring clothes for men, women, and children. Retailers in the United States worked with manufacturers to study Paris's trends and reinterpreted them to suit the needs of their own customers.
Historical Dictionary of the Fashion Industry. Francesca Sterlacci and Joanne Arbuckle.