To summarise Alexander Girard in one word, I would say color. I would then say that he was also a follower of Mid-Century Modern ideals but at the same time a traditionalist who was able to soften the edges of Modernism to produce design that was unique amongst most of his contemporaries and so timeless that his work is still sold today.
Alvin Lustig is best known for his book covers, but he also worked on his own modular typeface in 1939 entitled Euclid, always following his principles of simplicity and geometry. He also worked with Paramount Furniture to create the Lustig Chair, circa 1950. I talk about the Alvin Lustig book covers and how he invoked a subjective interpretation to Mid Century Modern graphic design.
Saul Bass was a man of his time, designing in a minimalist mid-century modern style that often reduced the essence of his subject down to the abstract idea in both his movie posters and his film title sequences.