Read my comment again. I said “roughly” the same time period. And yes, they overlapped for about a decade and from the perspective of someone unfamiliar with art history and living a century later, that is very close.

WW-I was a huge event in the history of this world, and it is a pretty clear divider between those two styles of art and architecture.

I wouldn't call pre-WWI and post-WWI "roughly the same time period". Not by a long stretch.

The length of time between them is not relevant in this case. What is relevant is the events that occurred between them.

Like the pre-atomic age and the post-atomic age. The events themselves in Hiroshima and Nagasaki were relatively short-lived. But their impact on this world greatly exceeded the time immediately before those two events and the time immediately after those events.

No, it's not a "clear divider" between the two. It was more of a slow transition between Art Deco and Art Nouveau, which as I already stated, overlapped for nearly a decade.

This is all pretty clearly laid out in the historical literature that you yourself linked to. You don't seem to understand how art movements work; there is not a firm dividing line when one ends and the next begins.

Beyond that, the question was about why people mix the two. Your nit-picking about how we should divide history is irrelevant to the discussion.

Art Deco got its start in 1910, but it didn't take off until after WWI. I did read those pages, even though I've been a big fan of Art Nouveau since I met my wife in the mid-90s and she introduced me to the style, and I was a fan of Art Deco since I was a kid in the 1970s and 1980s.

And I lived in Brussels, Belgium for almost eight years, in a house that was built in 1910 (according to the numbers formed into the bricks), and I lived about two blocks away from Hotel Horta. We have several coffee table books on Art Nouveau and Art Deco, some of which you can no longer find.

So, I am very well acquainted with both styles and when they were common. I also make a point of reading articles at any links that I include for reference.

Well, then it's a little unclear to me what you are arguing against. That they aren't from "roughly the same time period" from the perspective of someone that isn't familiar with art history? Why would someone unfamiliar with art history make a distinction between early 20th century art movements? They both occurred in the early 20th century, unlike (as I used in my example) neoclassical and medieval styles, which occurred centuries apart.

This is the subject of the grandparent post that I was replying to. Yes, in actuality, they overlapped a bit, and Art Nouveau waned as Art Deco rose – although again, this is not so clear cut in other art fields as it is in architecture; e.g., jewelry or furniture.

WWI was a watershed event. A world-changing event. It wasn't as short as the time period between when the first atomic bomb was dropped and when the second one was dropped, but in the scale of world events, it was still relatively short.

And while Art Deco got started before WWI, it didn't really take off until after WWI.

Just like Art Nouveau that got started in the 1890s, but didn't really take off until after the International Paris Exhibition in 1900.

Both styles took several years to get started and really take off. And Art Deco didn't really take off until after WWI.