I can explain this, but you have to start with the understanding that this entire thing is a gigantic in-joke of a piss take. This is going to be long.
First, you need to understand about ornamentation. Ornamentation is anything in a building that is basically a slightly superfluous detail.
In this colonial revival house (which is supremely balanced and has very clean lines), you can notice how the bottom windows have these clean ornamentations at the top, the way the columns fan out into a small design; the way the dormer windows have their own different style of decor complete with arch and keystone! That’s the ornamentation, it’s the small touches of structural decor. The majority of the time, they were there because they were needed to support something, to give additional support
Modernism changes that. The arrival of concrete and steel on architecture means you can explore structures that were never possible before, ways of getting light into a room that were never possible before, shapes that were never possible before; it basically heralds a new era entirely. For instance, Louis Sullivan’s National Farmer’s Bank of Owatonna, though a late entry into modernism (1908!):
Look how none of the voids (windows and doors!) have any sort of ornamentation. There is some ornamentation around the corners, sure, and while the ornaments themselves are very baroque and refined, there’s also a textural element on the tiling itself being patterned. But that’s very up-close detailing, or very far away detailing. You end up with a mix of the shape and texture being where detailing is explored, less so the ornamentation of before. Importantly, none of that ornamentation is, in any way, shape, or form, anything that is fundamentally structural. It’s become nearly superfluous.
And this keeps developing and developing and you arrive at things like skyscrapers. Sullivan may have been the father of the skyscraper, but I can think of no better follower than the trio of Shreve, Lamb & Harmon, who are most notable for the Empire State Building, but 500 5th Avenue may be the most direct example of what I’m talking about:
This modern-day ziggurat is almost all shape - the mullions (those vertical lines dividing windows) are largely decorative, and the ornamentation is very minimal and only serves to bring forward the shapes - notice how they only exist in what’s essentially the ceiling of each floor!
So we’ve established that ornamentation is steadily going away and no longer en vogue because architects are exploring the limits of shape itself, and they’re exploring unusual textures. But fast forward some 50 years, and this has become the singular architectural style that even exists. And a trio (Denise Scott Brown, Robert Venturi, and Steven Izenour) go to Las Vegas on a trip and come back with post-modernism. The idea is that buildings are either decorated sheds (ornamented houses) or ducks (buildings where the shape itself is the draw). The duck is a bit of joke to Americana - they passed by a duck building where the entire point was that it was a duck. There’s a disagreement, but even among the detractors, you’re going to see a more humorous take on Modernism. They’re going to make buildings that resemble other aspects of buildings, or other buildings, or whatever. It’s extremely in-jokey. It’s amazing.
Venturi and Scott Brown’s first major work is the Guild House, which is an apartment for the elderly. See if you can spot the joke:
Did you get it? The entire 5-story building is topped off with a colossal arch, treating the balconies like a void that you have to add an ornament on top. It’s a call back to the windows that we saw on the colonial house! This is a joke for a specific audience, but goddamn it’s really funny.
So the post-modernists are basically gonna set up jokes with architectural elements and play with aspects of it. It’s architecture for architecture nerds. It’s so obviously trying to be clever, and I love it.
Which brings us back to 550 Madison Avenue, by Johnson and Burgee, at the top of this post. The circle isn’t just the circle. It’s the entire slope and circle. The thing crowning the building. And you’ve seen it above doorframes and windows in a number of places.
The thing atop this dormer is called a pediment. It’s that mini roof. In this case we have a standard apex (the top) and a broken base (the bottom). This means that the top is connected and doesn’t recede to let in any ornamentation, but the bottom is broken up into two parts to let in the ornamentation.
On top of this door, you have a pediment broken on the apex. It’s filled in by that egg-like thing.
But what if you put a gigantic broken pediment one with no ornament on top of a building?
And there we have it. 550 Madison, a gigantic, supremely large scale shitpost, brought to you by technological advancements in construction and shifting design philosophies. “This skyscraper is structured like a window” is a really funny gag to pull if you’re the kind of person who actively has the same degree of architecture nerdery that I do. And architecture is one of the most common forms of art that you can observe and pull apart on your daily life.