Marilyn Loden hadn’t planned to make history when she spoke on a panel at the 1978 Women’s Exposition in New York.

She wasn’t even supposed to be there.

Loden, then 31 and a mid-level manager at New York Telephone Co., was asked to attend the exposition after the company’s only female vice president couldn’t make it. Four other women joined Loden on the panel titled “Mirror, Mirror on the Wall.” The name was fitting, Loden recalled, because the discussion centered on how women, and their self-image, were to blame for their lack of advancement in the workforce.

When Loden’s turn came to speak, she thought about how she had been tasked with at her company to explore why more women weren’t entering management positions. She had gathered enough data that she felt confident the problem extended beyond what her colleagues were wearing or saying.

“It seemed to me there was an invisible barrier to advancement that people didn’t recognize,” Loden said.

That day, she called it the “glass ceiling.”