During the 1920s and 1930s, people began to shift from “taking in the water” to “taking in the sun”, at bathhouses and spas, and swimsuit designs shifted from functional considerations to incorporate more decorative features.
Rayon was used in the 1920s in the manufacture of tight-fitting swimsuits, but its durability, especially when wet, proved problematic, with jersey and silk also sometimes being used.
Burlesque and vaudeville performers wore two-piece outfits in the 1920s. The 1929 film Man with a Movie Camera shows Russian women wearing early two-piece swimsuits which expose their midriff, and a few who are topless. Films of holidaymakers in Germany in the 1930s show women wearing two-piece suits, In 1928, Speedo introduced their racerback silk suit that was optimized to fit the body shape.
By the 1930s men began to go without shirts for swimming, and bare-chestedness in male swimwear became the norm by the end of the 1940s, including in competitive swimming events. The norm of male bare-chestedness in swimwear did not extend to other competitive sports.