A garden tub is a free-standing tub. It doesn't need to be triangular or placed in the corner, but it can be.
History and pictures here:
https://www.realtor.com/advice/home-improvement/what-is-a-garden-tub/
What is a garden tub? Also called a soaking tub, it's a free-standing, oval-shaped tub that's far larger, deeper, and more comfortable than the usual model—and a luxury bathroom amenity that's quickly gaining steam.
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Garden tubs first came into popularity in 18th-century Europe, and that's where their confusing name originates. ...
In the 1700s, "the wealthy often enjoyed soaking in their tubs in the privacy of their beautifully manicured grounds where they could appreciate the outdoors," explains Ana Cummings, an interior designer at Ana Interiors. "Eventually the tub made its way inside; however, they were still situated in a way where a full view of the landscape was achieved."
Until a few years ago, large shower spaces were a hot bathroom trend, but garden tubs are slowly but surely taking over.
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A standard garden tub is 42 inches wide, 60 inches long, and 24 inches deep. But they can come in any size and are often much larger—as much as 70 inches wide and twice as deep as a conventional tub.
It can be more challenging to install than a traditional tub, too. "A garden tub is much larger and is often free-standing with no walls around it," says Cummings.