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Jackie Kennedy is one of America’s enduring style icons for good reasons. The former First Lady of the United States was able to turn something as basic as a turtleneck and slacks into a stylish, chic ensemble and a simple strand of pearls into a must-have accessory. At least one of her fashion tips, though, was far less obvious and far more practical.

Kathy McKeon was just 19 years old and a recent immigrant from Ireland when she was hired for one of the most glamorous jobs imaginable—personal assistant to former first lady Jackie Kennedy. She served as Jackie’s assistant from 1964 through 1977, and she writes about her experience in her new memoir, Jackie’s Girl: My Life with the Kennedy Family.

As Jackie’s assistant, who lived in Jackie’s Fifth Avenue apartment in New York City, she was privy to many of the family’s secrets. While she always carefully guarded their privacy, in her memoir, she did let slip a few interesting facts, including noting that one of Jackie’s legs was a quarter of an inch shorter than the other. That’s not a huge difference, but it can lead to lower back pain and discomfort. Jackie, though, had a clever fix for the problem—nearly invisible lifts in each of her many, many shoes.

WATCH: 8 Beauty Lessons We Learned From Jackie Kennedy

Part of McKeon’s job was to oversee Jackie’s wardrobe. “I had never seen such a dazzling selection of shoes! London-look boots, pumps in every color, spotless sneakers for morning jogs around the reservoir,” People quoted from McKeon’s book. The incredible variety of shoes all had one thing in common, though: each had a quarter-inch lift affixed to its heel, “apparently to compensate for one leg being slightly shorter than the other.”

"No one would have ever guessed," McKeon wrote. It’s not surprising that even the most eagle-eyed Jackie watchers could have missed it, as a quarter-inch is an incredibly small difference, akin to a grain of rice. It just shows that Jackie really had an eye for detail.

As for where the idea for the minute lifts came from, RD.com suggests Jackie may have been inspired by her husband, John F. Kennedy, who also had one leg shorter than the other and wore heel lifts to help resolve his debilitating back pain.