Peggy Olson, that emblem of the pioneering ad woman in a man’s world on the television show “Mad Men,” would have been all of 8 years old on the night in 1947 when the real-life copywriter Frances Gerety coined the phrase “A Diamond Is Forever.”

As Ms. Gerety recalled in a 1988 interview with a co-worker, Howard Davis, she had just finished a series of ads and was headed to bed when she realized that she had forgotten to create a signature line. Exhausted, she said “Dear God, send me a line,” and scribbled something on a slip of paper. When she woke up and saw what she had written, she thought it was just O.K. A few hours later, she presented her idea at a meeting. According to her, “Nobody jumped.”

When Ms. Gerety applied to work at the Philadelphia advertising agency N.W. Ayer & Son in 1943, she was told that her timing was perfect: the agency had just lost a female copywriter. At the time, women were usually hired to write for women’s products only. Her main account would be De Beers. For the next 25 years, she wrote all of the company’s ads.

Her counterpart in publicity was Dorothy Dignam, a plucky brunette who kept a list of questions male co-workers asked her in the drawer beneath her typewriter; things she was meant to know as a woman, like, “Could a winter hat have a bird’s nest on it? Is Macy’s singular or plural? What do you give a girl graduating from a convent? Is this thing an inverted pleat?”