Sonia, Robert and their son Charles returned to Paris permanently in 1921 and moved into Boulevard Malesherbes 19.[21] The Delaunays' most acute financial problems were solved when they sold Henri Rousseau's La Charmeuse de serpents (The Snake Charmer) to Jacques Doucet.[22] Sonia Delaunay made clothes for private clients and friends, and in 1923 created fifty fabric designs using geometrical shapes and bold colours, commissioned by a manufacturer from Lyon.[23] Soon after, she started her own business and simultané became her registered trademark.
For the 1923 staging of Tristan Tzaras play Le Cœur à Gaz she designed the set and costumes.[24] In 1924 she opened a fashion studio together with Jacques Heim. Her customers included Nancy Cunard, Gloria Swanson, Lucienne Bogaert and Gabrielle Dorziat.[25]
With Heim she had a pavilion at the 1925 Exposition Internationale des Arts Décoratifs et Industriels Modernes, called boutique simultané.[26] Sonia Delaunay gave a lecture at the Sorbonne[27] on the influence of painting on fashion.