In the early 1900s there was a huge market for erotic postcards. Most of them were produced in France. The models often are found in classical poses, and were photographed in the best lighting possibilities available. There was an obvious need for the photographers to produce fine art, whereas their results were looked at as we look at contemporary porn. Showing a bit of leg was enough for a healthy Frenchman of that era to get aroused, so imagine the effect of looking at those beautifully photographed poitrines.