Difference between Rectangular body iMac and tapered?

Perhaps "thick case" and "thin case" are better design descriptors, as all iMacs are roughly rectangular when viewed from the front.

The thick case on a stand appeared in the G5 iMacs in May 2005.

The thick case, with minor variations in design and materials (first plastic, then aluminum), persisted through the Mid 2011 models:

The thin-case design appeared in Late 2012:

No thin-case iMacs have a built-in opticals drive. The thin case means that, as BDAqua mentions, three cooling fans were replaced with one. In the 21.5-inch models there was not room for a proper 3-5-inch desktop class hard drive so Apple opted for a laptop-class 2.5 in drive—not fast. The 27-inch models retained the 3.5-inch drive of previous models and expanded solid-state drive (SSD) options..

In 2021 both the 21.5 and 27 inch iMacs were discontinued and replaced by a new 24-inch design that was even thinner:

The MacTacker database app, free in the Mac App store, has history, specs, and images on almost every Apple product every built.

https://apps.apple.com/us/app/mactracker/id430255202?mt=12

BDAqua wrote:

The Tapered iMacs dropped from 3 fans to 1 fan, dropped possibility of using 3.5" HDDs, were generally stuck with crazy slow 2.5" HDDs, are next to impossible to upgrade RAM on the 21.5" iMacs, Replacing internal drive is a nightmare, the thin ones eventually got USB3 & TB3.

What else do you nneed to know?

With a few of the thin 21.5" iMacs, the RAM is not just sealed in, but soldered in.

I don't believe that any of the thin iMacs have FireWire. All have USB 3.0 or better. Apple changed from (USB 2.0, FireWire 800) to (USB 3.0, no FireWire) when they introduced the first thin iMacs in 2012. Thunderbolt 3 appeared in the 2017 models.

The non-Retina 13" and 15" Mid 2012 MacBook Pros had the then-desirable combination of USB 3.0, Firewire 800, and Thunderbolt 1. The Late 2012 Mac mini and Mac mini Server also had that desirable combination. They were the exceptions to the "you can have USB 3.0, or FireWire, but not both" rule.

I believe one advantage of the thin iMacs was supposed to be reduced reflections off the screen, due to the use of full lamination and anti-reflective coatings. Photographers sometimes compared the glossy displays on the earlier 21.5" and 27" iMacs unfavorably to panes of glass.

The Tapered iMacs dropped from 3 fans to 1 fan, dropped possibility of using 3.5" HDDs, were generally stuck with crazy slow 2.5" HDDs, are next to impossible to upgrade RAM on the 21.5" iMacs, Replacing internal drive is a nightmare, the thin ones eventually got USB3 & TB3.

What else do you nneed to know?

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Solid state options were available with the 2013 models - 21.5” included.

Also, thinness isn’t specified by Apple. IMO the 24” are thicker than their previous version.

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