 Once development began again in the 1950s and 1960s, the skyscraper entered a different phase of development, usually called the international or modern period.Some experimental designs in this style had been built in the U.S. using European architectural concepts in the early 1930s, most notably the Philadelphia Savings Fund Society and McGraw-Hill Buildings.Post-war, their modernist themes were used in a new generation of skyscrapers.