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In 1962, Cleveland Republic Steel suggested to the Steelers that they adopt the Steelmark logo as their official logo. The Rooneys like the idea, and soon, the the Steelmark logo that belonged to the American Iron and Steel Institute also belonged to the Steelers.
They wanted to put it on their helmets, but the fashion-conservative Rooney family, not wanting to to do anything too flashy or daring (understandably, since we're talking about Pittsburgh here, where they've only just now started to believe that people without moustaches could be decent, trustworthy people), experimented first with just putting the logo on one side.
That way, if the people of Pittsburgh turned out to not be ready for such a fashion-forward move, they could just, I don't know ... move to other side of the stadium and look at the blank side.
Myself, I rather like the three touching hypocycloids. They make use of the three primary colors, the very bedrocks of our visual abilities, and there's meaning behind each hypocycloid, too.
Anyway, the fans didn't object, and the Steelers followed the adoption of the logo with two other tweaks: They changed their helmets from gold to black, and they got the American Iron and Steel Institute to let them put "Steelers" in the logo, as opposed to just "Steel." That happened in 1963, and since then, things have remained unchanged.
You combine that kind of history and consistency with the overwhelming amount of quirk in the Steelers helmet, and I do believe you've got an undisputed champion here. It's quirk city on the side of Big Ben's head. Observe:
• It's the only professional helmet to use a logo that was once a corporate logo before it became a football team logo
. It is the only helmet in the NFL to have a logo on one side, but not the other.
• There are two colors present in the helmet logo that exist nowhere else in the Steelers uniform: red and blue.
It's extremely unique, it's got an interesting backstory, and obviously, there's a lot of winning tradition behind it. It succeeds on every front, and is something that's kind of evolved organically out of the town itself to become a powerful and iconic sports logo.
Thankfully for the local football team, Pittsburgh was, in the 60s, a city known for the hard, durable, solid steel it produced. If Pittsburgh had been a city known for cotton, latex, or their burlesque shoes, franchise history could've been totally different.