It’s an assignment that any developer today would be thrilled to take on: Program the world’s first modern computer — completely from scratch.

In 1943, six women were recruited to figure out how to program the Electronic Numerical Integrator and Computer, a.k.a. “the ENIAC,” a massive computing machine commissioned by the U.S. Army that was used to calculate ballistic missile trajectories during World War II.

The women — Betty Holberton, Jean Jennings Bartik, Kay McNulty, Marlyn Wescoff Meltzer, Ruth Lichterman, and Frances Bilas Spence — worked as “computers,” a clerical job that involved solving complex equations that would be used to build firing tables for guns. (Yes, their job title was literally “computer.”)  

Keep in mind, this was long before the hundreds of programming languages that we use today even existed. And it was certainly before a time when you didn’t need to know math to code. Though these women were highly trained mathematicians, they had no blueprint to follow when it came to ENIAC. They were given wiring and logical diagrams of the massive machine, and told to figure it out.

“When the request was made for people to volunteer to work on the ENIAC, I didn’t know what it was,” Bartik, one of the programmers (who died in 2011), recalled in a 1973 interview with the National Museum of American History. “But I had enough confidence in my own ability that if I could get on the ground floor in some area that I could do very well.”

When the ENIAC was finally unveiled to the public in 1946, there was no mention of the women who made it run and effectively invented modern programming. A 1946 New York Times article announcing the “amazing machine,” only credits the male inventors John Mauchly and J. Presper Eckert.

While figures like Ada Lovelace and Grace Hopper are widely recognized as pioneers of computer science, the ENIAC Six (as the women who programmed the ENIAC became known) were almost erased from history. So in honor of Women’s History Month, here’s what you need to know about the oft forgotten legacies of some of the most influential women in programming, plus how their contributions laid the foundation for future innovations.

Programming a computer with zero instructions

With the ultra-sleek computers and razor-thin mobile phones most of us use today, it’s tough to comprehend the sheer size of the ENIAC. Housed at the University of Pennsylvania Moore School of Electrical Engineering in Philadelphia, the ENIAC was 8 feet tall and 80 feet long, and weighed 30 tons.

The ENIAC contained 40 panels that were each about the size of a commercial refrigerator, lined up in a U-shape, with 18,000 vacuum tubes each. “We knew we were supposed to run the machine and set up problems for the machine, but no one had any techniques or anything,” Bartik said in the 1973 interview.

At its core, the ENIAC was a very advanced series of calculators that were wired together to flow information from one machine to another.

Not only did the women have to translate their calculations into steps that the ENIAC could handle, but they also had to literally wire the machine, said Kathy Kleiman, historian, author, and founder of the ENIAC Programmers Project, in a 2018 TEDx Talk. “They had to track each piece of data, wire it into a panel, such as a multiplier or a ‘square rooter,’ and then move the result — physically by wire — to another panel for storage,” she said.

The women kept track of the steps and diagrams for the wires manually on what they called “pedaling sheets.” As Bartik described it: “ENIAC was a son of a b— to program,” she said in the documentary “The Computers.”

But the tedious work it took to operate ENIAC was nothing compared to what “computers” had to do by hand. A single differential calculus equation could take 40 hours to complete using a mechanical desktop calculator, but the ENIAC could complete it in minutes.

Honoring a lost legacy

The ENIAC is “the great-great-grandfather of everything on your smartphones and in our laptops,” Kleinman said in the TEDx Talk. The ENIAC was the precursor to the ENAC, which ultimately led to the development of the UNIVAC, the first commercially-produced computer.  

More than 75 years after the ENIAC was released, historians agree that the ENIAC Six didn’t get credit for all that they did for computer science with their work on ENIAC and beyond. Here are some of their post-ENIAC accomplishments:

• Jean Bartik joined the teams that programmed the Binary Automatic Computer (BINAC), and then was in charge of logical design for the UNIVAC.
• Betty Holberton ran the committee that created the programming languages COBOL and FORTRAN.
• Kay McNulty was involved in early research that was testing the possibility of a hydrogen bomb.
• Ruth Teitelbaum taught programming on the ENIAC when it was relocated to the Aberdeen Proving Ground in Maryland.
• In 1997, the ENIAC Six were inducted into the Women in Technology International Hall of Fame.

One explanation for why the ENIAC Six faded into obscurity? At the time, their work was “viewed as not interesting, because nobody understood programming,” says Mitch Marcus, an ENIAC historian and emeritus professor at the University of Pennsylvania. While people marveled at the engineers who built the ENIAC, they simply couldn’t appreciate the undertaking that the ENIAC Six took on to get it to actually work.

Today, their impact is clear: “They invented the idea that you could write programs,” Marcus says. Next time you write a line of code, run a complicated program, or find a bug, know that it’s in part thanks to the six women who were given the massive responsibility to program the ENIAC more than 75 years ago. At the very least, be grateful you didn’t have to do 40 hours of calculus before you could even start programming.