What are the differences between the displays used in the "Tapered Edge" Aluminum iMac models?

All "Tapered Edge" Aluminum iMac models -- regardless of series -- have an LED-backlit 16:9 widescreen TFT active matrix display with IPS technology, but the dimensions and native resolutions are different.

Photo Credit: Apple, Inc. (21.5" & 27" Tapered Edge iMac, Left & Right, Respectively)

Specifically, all of the pre-"Late 2015" 21.5-Inch models have a native resolution of 1920x1080, whereas the non-Retina 27-Inch models have a native resolution of 2560x1440. The "Retina 4K" 21.5-Inch models have a native resolution of 4096x2304 and the "Retina 5K" models have a native resolution of 5120x2880.

However, by default, the Retina 4K models run "pixel doubled" at 2048x1152 and the Retina 5K models likewise run at a resolution that looks like 2560x1440. However, these "pixel doubled" settings look significantly sharper than earlier displays at the same resolution.

It also is worth noting that the "Mid-2017" and "2019" Retina 4K and Retina 5K models are brighter than the displays in previous models with a brightness rating of 500 nits and support for wide color (DCI-P3). They support one billion colors.

Apple reports that the "cover glass is fully laminated to the LCD and an anti-reflective coating is applied" for all of these models, too.

What type of video processor is provided by the "Tapered Edge" Aluminum iMac systems? Which have dedicated memory? Which have integrated memory?

The education-only iMac "Core i3" 3.3 21.5-Inch (Early 2013) as well as a variety of subsequent models have cheaper "integrated" graphics that "borrow" memory from the system. Other models have "dedicated" video memory.

However, the integrated video solutions vary significantly as do the dedicated video options.

Specifically, these models have integrated memory and are best to be avoided by those who place maximum importance on video performance:

The below models have dedicated video memory and generally provide better graphics performance than systems with integrated memory released at the same time:

The video processor is affixed in place and cannot be upgraded on any of the "Tapered Edge" Aluminum iMac models after purchase.

Do the "Tapered Edge" Aluminum iMac models support multiple displays? What is the maximum resolution for an external display?

Yes, all "Tapered Edge" Aluminum iMac models support multiple displays in addition to simultaneous support of the internal display at its native resolution.

Specifically, all pre-Retina "Tapered Edge" iMac models -- the Late 2012, Early 2013, Late 2013, and Mid-2014 lines -- support two external displays up to 2560x1600 via Thunderbolt.

The Late 2014/Mid-2015 Retina/5K models -- the iMac "Core i5" 3.5 27" (Late 2014), "Core i7" 4.0 27" (Late 2014) and "Core i5" 3.3 27" (Mid-2015) -- formally support a single external 3840x2160 (4K UltraHD) display via Thunderbolt 2. However, third-parties have discovered that these models also can support two simultaneous 27-Inch Thunderbolt displays at 2560x1140 each instead of a single 4K UltraHD display.

The entry-level 21.5-Inch "Late 2015" iMac -- the iMac "Core i5" 1.6 21.5-Inch -- supports a single external display up to 3840x2160 resolution whereas the other 21.5-Inch "Late 2015" models support a single external display up to 4096x2304. The 27-Inch "Late 2015" iMac models, on the other hand, support a single external 5120x2880 dual-cable external display or two external 4096x2160 displays.

All "Mid-2017" and "2019" iMac models, regardless of display size, support one 5120x2880 (5K) external display at 60 Hz with support for one billion colors; two 3840x2160 (4K UHD) external displays at 60 Hz with support for one billion colors; or two 4096x2304 (4K) external displays at 60 Hz with support for millions of colors.

The 27-Inch "2020" iMac models support up to one 6016x3384 (6K) external display at 60 Hz with support for 1 billion colors; one 5120x2880 (5K) external display at 60 Hz with support for 1 billion colors; two 3840x2160 (4K UHD) external displays at 60 Hz with support for 1 billion colors; or two 4096x2304 (4K) external displays at 60 Hz with support for millions of colors.

Can you wall mount the "Tapered Edge" Aluminum iMac models? Are they VESA compliant?

As shipped by default, the "Tapered Edge" Aluminum iMac models are not VESA compliant and are not possible to wall mount.

However, Apple subsequently started offering the "Late 2012" iMac models -- and has offered all subsequent models, including the current ones -- with a "Built-In VESA Mount Adapter" as a US$40 option at the time of purchase only. This VESA option does not include the traditional iMac "foot" stand, though.

Photo Credit: Apple, Inc. (iMac Retina 5K with VESA Mount Adapter Option)

Unfortunately, there is no official way to convert one of the "Tapered Edge" iMac models that shipped with the stand into a VESA compliant model.