After 67 Years of Marriage, What Do Jimmy and Rosalynn Carter Mean by 'ILYTG'?

Rosalynn and Jimmy Carter

Martin Schoeller

By Sandra Sobieraj Westfall

10/08/2014 AT 11:15 AM EDT

When Jimmy Carter was in the White House, the hashtag was still a lowly pound sign, and Twitter was decades away.

But the former president, now 90, proved he was a man ahead of his time when, as a smitten young Naval officer in the 1940s, he coined the acronym "ILYTG."

" 'I love you the goodest.' That's what my mother and daddy used to say back and forth, and I picked it up with Rosa," the nation's 39th president tells PEOPLE in an exclusive joint interview with his wife of nearly seven decades, Rosalynn Carter.

The couple attribute their longevity to giving each other space, as well as their shared passion for service: They still travel around the globe – to build houses in Nepal, or to fight river blindness in South Sudan – with the Carter Center.

The monogram is inscribed on a compact he once gave his bride, the future first lady, that today is on display at the Jimmy Carter Presidential Library and Museum in Atlanta.

"Now all our children do the same thing – ILYTG on the phone or in emails. They generally just put the initials," Jimmy Carter said. "And sometimes they change the initials and make you guess what they're talking about." LOL.

For our at-home interview with the Carters, pick up this week's issue of PEOPLE, on newsstands Friday