In conversation, some things are better left unsaid – and in the Historic District, some things are better left unbuilt.

That’s the unspoken commentary behind a new exhibit that opened at the National Building Museum this past weekend. “Unbuilt Washington” explores the monuments and buildings – and occasional Venetian-style canal – that might have graced the District, if only they had made it past the drawing board.

There’s the colossal pyramid honoring Abraham Lincoln, or the medieval-style Memorial Bridge dedicated to Ulysses S. Grant, or the new executive mansion built further up 16^th Street, atop Meridian Hill.

As a D.C. guide, I thought this exhibit was a pretty fascinating tour of an alternate Washington. And with Thanksgiving approaching, I was also left feeling grateful that some of these outlandish structures were left unbuilt – either through lack of funds, shifting priorities, or public outcry.

The exhibit reminds you that from its inception, the federal capital has presented a veritable blank slate for architectural imaginations and often competing notions of national expression.

“Unbuilt Washington” starts with French architect and engineer Pierre Charles L’Enfant’s iconic designs for the city. He envisioned a grid of streets overlaid with diagonal avenues, springing from 15 plazas dedicated to each of the states (the original 13, including new additions Vermont and Kentucky).

Not all of his plan was built, but the city we ended up with is much closer to L’Enfant’s vision than the one proposed by Thomas Jefferson. As Secretary of State, Jefferson suggested buillding a capital that was only 11 blocks long and 3 blocks wide – which would have assured that much of Washington was indeed left unbuilt.

L’Enfant was eventually let go over artistic and personality clashes, the first in a long line of spurned designers in the city’s history. But there were quite a few amateur architects clamoring to take his place, and to design the thousands of buildings desperately needed to fill out his well-laid streets.

Chief among these buildings were the President’s House and the Congress House, whose designs were chosen by national competitions. As the first two major structures built in Washington, they are so iconic that it’s hard to realize that their final looks were far from inevitable.

It’s equally hard to imagine in their place some of the also-rans that didn’t make the cut.

In the exhibit you can see an anonymous design for the President’s House attributed to Jefferson and modeled after one of his favorite Palladian villas. The winning design – the familiar White House we know today – was actually modeled

after a private home in Dublin called Leinster House, which ended up becoming Ireland’s parliament building.

For our own Congress building, there was no shortage of submissions, sporting a wide variety of styles and competing domes. One of my favorites (by which I mean least-favorite) was topped by a rather awkward rooster weather vane.

Over the years, the city’s official architecture began to take root, with Jefferson’s favored neo-classicism as the dominant style. Still there was constant experimentation, with new buildings designed in all styles, shapes, and sizes.

Some of the more ambitious designs began to elicit public backlash, both from Washingtonians and members of Congress. After D.C. architect T. F. Schneider built the 160-foot Cairo Hotel in residential Dupont Circle, the D.C. Board of Commissioners responded by passing Washington’s first height limitations.

A few years later, Schneider again reached for the sky by proposing a 250-foot office building for downtown Washington. Congress stepped in and passed the Height of Buildings Act of 1910, which ensured that Washington would stay relatively grounded.

But some of the designs featured in the exhibit make you wonder if the alternative would have been better than the real thing.

I quite liked a rounded version of the Kennedy Center, which seems to fit the curve of the Potomac’s banks. The terrace would have actually allowed theater-goers to walk down to the river rather than hover above it, and even arrive and depart by boat.

As the Washington Post notes, it also sported a circular “central lobby space that would have been much more congenial and social.”

As you walk through “Unbuilt Washington,” you’ll see alternate Smithsonian museums and galleries, and drastically different versions of the National Mall, first conceived by L’Enfant two hundred years ago. There are schemes to submerge the Mall underwater, re-plant and re-forest it, and expand it to the east and to the south.

One proposal would move the Supreme Court all the way down South Capitol Street to the shores of the Anacostia. The new Nationals ballpark in that location has pretty much assured that this won’t happen.

There are plenty of other designs for Washington’s waterfront that have gone unrealized, including various plans for

an aquarium to be built closer than Baltimore’s Inner Harbor. Any of these would be better alternatives to our existing National Aquarium, which is relegated to the basement of the Department of Commerce Building.

And speaking of aquatic life, how about the notion of watching dolphins frolic on Massachusetts Avenue? If the proposed Dolphin Hotel had been built at Mass Ave and I Street in the 1980s, D.C. might have rivaled Sea World.

That’s one of the most entertaining unbuilt designs in the entire exhibit. But just because it seems absurd, doesn’t mean that it couldn’t have been built.

The ultimate take-away lesson from this exhibit: Even in a monumental city like Washington, D.C., nothing is truly set in stone.