In 1964, Jaffee created his longest-running Mad feature, the Fold-In. In each, a drawing is folded vertically and inward to reveal a new "hidden" picture (as well as a new caption). Originally, Jaffee intended it as a one-shot "cheap" satire of the triple fold-outs that were appearing in glossy magazines such as Playboy, National Geographic and Life. But Jaffee was asked to do a second installment, and soon the Fold-In became a recurring feature on the inside back cover of the magazine. In 2011. Jaffee reflected, "The thing that I got a kick out of was... Jeopardy! showed a Fold-in and the contestants all came up with the word they were looking for, which was “Fold-in.” So I realized, I created an English language word."
In 2010, Jaffee described the earliest Fold-Ins:
- I thought to myself... now it's folded in and I've got to have something on the left side here, and something right side here. And the only thing that popped into my head was that Elizabeth Taylor had just dumped Eddie Fisher and was carrying on with Richard Burton. So I had Elizabeth Taylor kissing Richard Burton, and a cop is holding the crowd back—and just for the fun of it I put Eddie Fisher being trampled by the crowd. What a cruel thing to do! And then, when you fold it in, she's moving on from Richard Burton and kissing the next guy in the crowd. It's so simplistic and silly and juvenile! And anyone could have done that!
- I showed it to Al Feldstein, and the first thing I said was, "Al, I've got this crazy idea, and you're not going to buy it, because it mutilates the magazine." So I put it in front of him, and the thing about Al was, he liked things that intrigued him. The mechanics of it intrigued him. He said, "You mean, you fold it, like this...? And then...?" He folded it, he unfolded it, he folded it, and then he said, "I like this!" But I said, "Al, it mutilates the magazine." And he said, "Well, I'll have to check it with Bill." He takes it, runs it to Bill's office, and he was there a little while, and he comes back and he says, "We're going to do it! You know what Bill said? Bill said, 'So they mutilate the magazine, and then they'll buy another one to save!'
- Four or five weeks later, Al comes over to me and says, "When are you going to do the next Fold-in?" And I said, "I don't have another Fold-in. That was it!" So he said, "Come on, you can come up with something else." I wracked my brain, and the only thing I could come up with was Nixon . That one really set the tone for what the cleverness of the Fold-ins has to be. It couldn't just be bringing someone from the left to kiss someone on the right."
The Fold-In has since become one of Mad's signature features, and has appeared in almost every issue of the magazine from 1964-2008. A single issue in 1977 was published without a Fold-In (though Jaffee supplied the issue's back cover), and a 1980 issue featured a unique double-visual gimmick by Jaffee in which the inside back cover and the outside back cover merged to create a third image when held up to the light. The third-ever Fold-In in 1964 featured a unique diagonal folding design, rather than the standard left-right vertical format. The image revealed the four members of The Beatles becoming bald (and thus losing their popularity).
The Far Side creator Gary Larson described his experience with the Fold-In: "The dilemma was always this: Very slowly and carefully fold the back cover... without creasing the page and quickly look at the joke. Jaffee's artistry before the folding was so amazing that I suspect I was not alone in not wanting to deface it in any way." In 1972, Jaffee received a Special Features Reuben Award for his Fold-Ins.
Jaffee only uses a computer for typographic maneuvers to make certain Fold-in tricks easier to design. Otherwise, all his work is done by hand. "I'm working on a hard, flat board... I cannot fold it. That's why my planning has to be so correct." In 2008, Jaffee told one newspaper, "I never see the finished painting folded until it's printed in the magazine. I guess I have that kind of visual mind where I can see the two sides without actually putting them together." Contrasting current art techniques and Jaffee's approach, Mad's art director, Sam Viviano, said, "I think part of the brilliance of the Fold-in is lost on the younger generations who are so used to Photoshop and being able to do stuff like that on a computer."