National Bellas Hess Midsummer Sale Catalog (1936)

These swimsuits look almost modern, and their wearers almost liberated—but oh dear, look at the materials they're made off!  All these costumes are made of knitted wool which was heavy, sagged when wet and took ages to dry.

Help was at hand, though, because the 1930s was the decade that saw the introduction of "lastex".  Lastex was an elastic fibre that was made with a core of rubber surrounded by wool, silk, cotton or rayon.  It was perfect for girdles and bras—and of course, swimsuits.  It made possible the sleek "glamour" swimming costumes worn by the pinups of the forties and fifties.

Meanwhile, "all wool" outfits like these continued to be made and worn into the early fifties.  Lastex costumes were not cheap, war interfered with production, and if all else failed it was still possible for a hopeful beach-goer to find a pattern and knit her own swimsuit to order.