Some designers changed fashion because they introduced a silhouette or idea that was adopted well beyond their own customers. Four examples, with dates.
Coco Chanel
She became a licensed milliner in 1910 and opened Chanel Modes at 21 rue Cambon. Chanel No. 5 followed in 1921, and her soft tweed suit, cut without stiffening or shoulder pads, is dated to 1923. She is also associated with popularising the little black dress, which American Vogue published in a version in 1926.
Christian Dior
His house was founded in 1946 and showed its first collection on 12 February 1947. The “New Look” had rounded shoulders, a cinched waist and a very full skirt.
Yves Saint Laurent
His 1965 Mondrian collection brought geometric shift dresses into fashion, Le Smoking, a women’s tuxedo suit, is dated 1966, and his first Rive Gauche ready-to-wear line was presented in September 1966.
Vivienne Westwood
With Malcolm McLaren she ran a shop at 430 King’s Road, London, that helped define the punk look. Her Pirate collection (1981) was their first shown to the press and international buyers. She received an OBE in 1992 and was made a Dame in 2006.
Sources: Wikipedia entries for Coco Chanel, Christian Dior, Yves Saint Laurent and Vivienne Westwood.