There are some 175,000 pieces in MoMA’s entire collection, and roughly 28,000 in the architecture and design section, which includes everything from some of the 20th century’s most famous cars and chairs to the archive of the modernist grandee Mies van der Rohe. New pieces can only be added after winning the approval of an acquisitions committee composed of 25 architecture and design specialists, who meet every three or four months.

The committee must be convinced that each addition meets the entry criteria. Does it excel in terms of form and function? Does it embody the values of clarity, honesty and simplicity that MoMA considers essential to good design? Has it has made an impact on our lives? Is it innovative? Then there’s the clincher. “If this object had never been designed or manufactured, would the world miss out?” said Paola Antonelli, senior curator of architecture and design at MoMA. “Even just a bit?”

How did the @ fare? Brilliantly, according to Ms. Antonelli. By giving that once obscure accountancy symbol a new application without distorting its original meaning, Mr. Tomlinson was deemed to have checked all of MoMA’s boxes in terms of form, function, values, cultural impact and innovation. She sees “snail,” “pig’s tail” and its other nicknames as proof of its importance, because we care so much about the @ that we’ve started to mythologize it.

Fair enough, you might say, but what does its transformation have to do with design? After all, the new @ looks exactly the same as the old symbol, and isn’t a physical object like the chairs, cars or architectural drawings that you’d expect to find in a design museum.

That’s exactly why MoMA admires it. First, both the old and new @ fulfill the same function of simplifying and clarifying something that’s fiendishly complicated to make and interpret: handwritten script and computer code respectively. Ms. Antonelli describes that as “an act of design of extraordinary elegance and economy.” Both qualities are prized by MoMA, especially “economy” in a time of recession and environmental crisis, when reinventing something that’s under-used seems much smarter than designing something new.

Timeliness matters to MoMA too, and the new @ is timely not only in its economy but also precisely because it is not physical (just like equally dynamic areas of contemporary design such as software and social design). “MoMA’s collection has always been in touch with its time,” Ms. Antonelli said, “and design these days is often an act with aesthetic and ethical consequences, not necessarily a physical object.”

That’s why MoMA decided against adding a specific version of the @ to the collection in favor of using it in different typographic styles and sizes. Ms. Antonelli likens it to the museum’s acquisition of “The Kiss,” a performance art piece by Tino Sehgal, in which a couple embrace for several hours. Just like the @, each performance can take a different form with new protagonists — though there is a difference. MoMA reportedly paid $70,000 for “The Kiss,” while the @ is joining the collection free.