Mac Headaches: External Monitors

Macs have been the "It Just Works" computer for decades, but connecting multiple monitors frequently creates questions or problems. I connected five monitors to a Macintosh IIx back in the 1990s, so what happened?! Back then it was very straightforward: each monitor required a separate video card and cable from the computer. A lot has changed and it can be complicated to connect multiple displays, now. Let's get into it.

• Your Mac May Vary
• The Simple Solution
• USB-C Isn't One Thing
• DisplayPort: USB v Thunderbolt
• Bandwidth Bummers
• Multi-Stream Transport
• DisplayLink Docks
• Docks and Hubs
• Other Articles

Your Mac May Vary

Different computers have different external monitor support, but this is true for Macs even more than most. As good as the Apple Silicon processor line is, it brings more variations. Even in the same model and year of computer (e.g. 2025 MacBook Pro), differences exist.

There are a number of Apple Support articles that can help you identify your computer, which then link to the Tech Specs for the different models. If you are considering purchasing a new or used Mac and have specific display requirements, these pages may help.

Identify Your Mac

You can start with the Identify the model of your Apple device article, or go to the specific type directly:

• Identify your MacBook Pro model
• Identify your MacBook Air model
• Identify your Mac mini model
• Identify your iMac model
• Identify your Mac Studio model

External Monitor Support

Apple has support articles for understanding how many external displays your computer supports. These are a good starting point, but there is a lot of additional nuance between these "base" specs and a specific desk setup, so keep that in min.

Monitor support varies mostly by processor type. The MacBook Pro can have a "base" M-series processor, or a Pro or Max chip. Each of these have different rules by processor type and generation, so an M2 and an M3 may not be the same.

For instance, the Apple Silicon MacBook Air supported only one external monitor until the MacBook Air M3 (March, 2024).

• How many displays can be connected to MacBook Pro
• How many displays can be connected to MacBook Air - M4-specific
• Use dual monitors with your MacBook Air and MacBook Pro with M3 chip

You'll probably notice most of these articles talk specifically about Thunderbolt rather than USB. We'll get to that distinction.

Closed Lid Limits

How you use the computer makes a difference, too. I bought a 2024 M3 MacBook Air, and it's great, but I wanted to use two external monitors. It supports that, but not how I expected. Some Macs only support multiple external monitors with the lid closed, like the one I own. As long as it has to run the internal display, it will power only one external. I have a setup where I use two displays with the laptop to the side for "auxiliary" content like email or Discord, but I couldn't do that with the M3 Air. It's a great computer, but that limitation led me to get a MacBook Pro M4 Max.

If you use your laptop with the lid closed – in a dock, tucked below the monitor, or just off to the side – you may be able to avoid the "Pro tax" while still getting two external displays.

It hopefully goes without saying, but in a closed-lid setup you need an external keyboard and pointing device.

Special Cases

We won't dig into them, but your Mac also supports some special cases, like using an iPad as a second screen with Sidecar or streaming to a television using AirPlay, where you can mirror or extend your desktop, again depending on the device. It's not currently clear to me how these work with the limit on monitors for specific devices, but I wanted to mention these possibilities.

The Simple Solution

Before we go too much further; most Macs support multiple external displays. You can usually accomplish that the same was I did back in the 90s – every monitor connected directly to the computer. This is generally fine on desktops but not ideal with laptops that sometimes leave the desk.

Still, if your Mac has the ports and hardware support, this is the most reliable solution. If you – like most people – are looking for a one-cable (or at least few cable) solution, read on.

USB-C Isn't One Thing

The biggest confusion comes from the USB-C port. The USB name covers both physical connectors, like USB-C, as well as data and power specifications.

If you're interested, Consolidated Cables has an article about USB ports and basic specs.

But wait, there's more! USB-C connectors also support "alternate modes" which let the same connector provide DisplayPort, Thunderbolt 3 or later, and other less-common signals.

Wikipedia's USB-C article is long but thorough if you want details. I recognize most people truly don't care, but it is good to know it's an option.

The three relevant protocols that a Mac's USB-C port can offer are USB, Thunderbolt, & DisplayPort. But there is still a lot of confusing detail.

DisplayPort: USB v Thunderbolt

Because the connector is the same, we have to be aware of the differences in the protocols. Most Macs provide Thunderbolt, but they support the DisplayPort Alt-mode over USB as well. And there are different rules for the two protocols, even if the port is the same.

Apple has an article to help you Identify the ports on your Mac so you know which standards your Mac uses. Be aware that while all Apple Silicon Macs offer Thunderbolt, not on all of the USB-C ports on some models are Thunderbolt, especially the front ports on the Mac mini and Mac Studio...depending on processor.

Just to make things confusing, the USB4 spec contains support for Thunderbolt 3/4, so USB4 devices may support Thunderbolt but they don't have to.

It really is complicated.

Chaining and Splitting

Thunderbolt mostly uses daisy-chaining, or connecting devices in series. Most Macs with Thunderbolt 3 or later have the ability to support two displays on one port, which is ideal for docking stations. There are limitations on this, so please reference the Apple Support articles above for your specific computer. I have not personally confirmed this, but it seems in some cases you can chain a DisplayPort monitor from a Thunderbolt monitor. This was not the case with the Apple Thunderbolt Display (using Thunderbolt 2 over a Mini-DisplayPort connector), and does depend on the Thunderbolt controller of the first display.

On devices which support chaining multiple monitors over Thunderbolt, at least the first monitor must have two Thunderbolt ports. Such monitors are usually from the professional display market and are often much more expensive than their DisplayPort counterparts – a quick search found only one for under $1000 USD – but they do exist.

Most of the time, though, monitors with a USB-C port are offering DisplayPort Alt-mode over USB. Macs support this, but with only one display per port – more accurately one display per chain*. This is where many of the headaches happen when people try to use a dock for one-cable convenience.

* I say one display per chain because you might be able to run two DisplayPort adapters off a USB-C hub or dock. I have not been able to find a reputable source for this, though. Most of the examples of this are using one of the solutions further down this article.

Bandwidth Bummers

When we start getting into specific docks, we can also run into the issue of supported bandwidth. The number of pixels each screen has (resolution), and the number of times per second they update (refresh rate) adds up to certain bandwidth limits. These concerns are more rare with modern docks, but Apple's tendency toward "Retina" high-resolution (generically called HiDPI) can make this a possible concern.

This is why the descriptions in the Display Support section on the Mac Tech Specs have so much detail:

M4 Max
Simultaneously supports full native resolution on the built-in display at 1 billion colors and:
Up to four external displays: Up to three external displays with 6K resolution at 60Hz over Thunderbolt and one external display with up to 4K resolution at 144Hz over HDMI
Up to three external displays: Up to two external displays with 6K resolution at 60Hz over Thunderbolt and one external display with up to 8K resolution at 60Hz or one external display with 4K resolution at 240Hz over HDMI

Your Mac will output the preferred resolution of the monitor, no matter what you select in the Displays screen; even choosing "low resolution" modes. Older macOS versions supported disabling HiDPI (Retina) modes, but those commands do not appear to work on more recent versions macOS.

So while there are some limits within the Mac itself, there may be lower limits on the docks or devices you use.

A Dell Community Forum thread from 2019 does an excellent job of explaining some of these details on the Dell WD19TB, where the internal design of the dock may be a limiting factor. I use 4k displays. Even when I select a scaled resolution, the Mac outputs the high resolution signal. This may have prevented me from testing whether I could get three separate displays working on my WD19TB simply because 4K requires more bandwidth.

Multi-Stream Transport

Some docks – and monitors – support DisplayPort Multi-Stream Transport which allow you to either daisy-chain multiple monitors together or use a dock that splits a single DisplayPort channel to multiple separate displays. This is fairly common in the PC world, and you can even find inexpensive devices that support four monitors on Windows, but it is completely unsupported by macOS.

Apple has never supported DisplayPort MST in the fifteen years it has been available, even when it was usable on Intel-based Macintosh hardware running Windows. As I understand it, Apple Silicon display controllers do not include MST support at all, so MST support cannot be added to current devices.

If you have a dual-monitor dock or a chain of monitors that works on Windows using DisplayPort MST, it will not work on macOS. Depending on the design, it may allow you to use one external monitor or mirror the same output on both displays, but there seem to be cases where multi-monitor docks do not work at all.

MST Documentation Problems

Awareness of MST is especially important if you are looking at docks originally focused on the Windows market. Most of the Windows world assumes MST support is available and it may not even be mentioned in the user guide. This inconsistency is one of the biggest risks as a Mac user looking for docks with multi monitor support.

I use a Dell Thunderbolt WD19TB dock for work. It drives my two external 4k displays – one over Thunderbolt and one over HDMI. The dock is advertised as supporting four displays, as the WD19TB manual indicates. After playing around with different ports and resolutions, I was able to get three external displays working, but only with mirroring the DisplayPort-over-USB-C to the second regular DisplayPort. The third (mirrored) monitor does not appear in the OS at all. This is a pretty common symptom of MST.

Despite no mention of MST or Multi-Stream Transport in the product specs or even the User Guide, a table in the W19TB Administrator's Guide section about firmware update timings mentions MST.

Thunderbolt Docks

There have been a variety of Thunderbolt docks available or years, but a Thunderbolt dock does not mean it is free of complexity. The afore-mentioned Dell WD19TB is one such, where despite Thunderbolt support it uses MST for some of the display connectors and has internal bandwidth limits.

When selecting a dock, check for Macintosh support. Brands like CalDigit and especially OWC are known for working with Macs.

DisplayLink Docks

An option that appeared well before MST and Thunderbolt is DisplayLink. Using software to create virtual displays and stream that information over USB, DisplayLink works around lacking hardware support.

DisplayLink requires a software driver to be installed on the operating system. It is widely supported on Mac, Windows & Linux, but it isn't plug-and-play. If you are on a work-managed device, you may need permissions to install the DisplayLink driver.

Some people have had negative experiences with DisplayLink, but that has not been the case for me. If you have a Mac with limited support for external monitors, it might be a useful alternative.

Not Without Limits

DisplayLink is an acceptable general computing solution. It uses software to simulate display support, meaning it leverages your processor to do the work. For most casual computer use – word processing, code editing, browsing the web – this may be a viable choice.

What it doesn't do well is motion, either gaming or video. The DisplayLink software layer causes increased lag to the display, and uses various compression methods which may limit the image quality.

While some of the DisplayLink systems do support higher frame rates, you should not expect it from these devices in general. DisplayLink also may have issues with DRM – Digital Rights Management – which is required to play most video at 1080p or higher quality. This means you may get a black box trying to view most video content on a DisplayLink monitor. Some users have found that disabling hardware acceleration in their browser resolves the issue. If you are able to stream content on DisplayLink, the image quality may be degraded by the limits of software processing and the USB bandwidth.

A 2020 post to the DisplayLink forums includes a response from a member of the support team clarifying that color profiles and some software display controls are not supported through DisplayLink. If color accuracy is important, this is probably not the solution you are looking for.

Identification

It may be difficult to distinguish an MST dock from a DisplayLink dock when comparing or shopping, but the specs usually mention DisplayLink. Multi-monitor docks that provide no information on their product pages are likely using MST.

As an example, the Targus DOCK430USZ makes no mention of either MST or DisplayLink on the product page. After locating the PDF Manual there's a small blurb that clarified it uses MST (complete with spelling error):

Graphic adapter supporting Multi-Stream
Transport (MST) Mode is reuqired for dual or tri monitor
display mode

Other Software-Required Alternatives

There are even more alternatives to the alternative. The less-known multi-monitor solution called InstantView by SiliconMotion, and another from j5create.

InstantView doesn't require installing a DisplayLink driver, you do have to run the InstantView software and grant it permissions. There are a handful of USB hubs from Hyper and Anker that supports a second monitor through "no install InstantView" software. It's still loading the second display driver in software, and using some OS tricks to do it. Experiences vary with performance, resolution issues, color inconsistency, and lag. A Reddit post to r/Mac comparing them in 2024 was not very complimentary, though.

j5create seems to use their own separate hardware and driver combination. Information was more limited on these products.

Docks vs. Hubs

We've identified a number of considerations for docks, but we need to talk about hubs. Unlike docks, which provide a variety of ports, hubs are almost exclusively focused on providing more of a specific type of port. This is the case with simple USB Hubs – one USB in, two or more USB ports out.

Some USB-only hubs support USB-C DisplayPort Alt-mode, allowing you access to regular USB ports without preventing you from using the USB-C capabilities. Hubs like these might be good for on-the-go use, but they are still limited to one DisplayPort output per port to the best of my ability to find information.

Thunderbolt Hubs

Thunderbolt standardized on the USB-C connector, and USB4 is essentially Thunderbolt 3. Thunderbolt 4 and 5 are mostly incremental bandwidth improvements. While actual Thunderbolt docks have been around for some time – I used a Thunderbolt 2 dock with my 2013 MacBook Air – many of these used Thunderbolt but did not provide multiple Thunderbolt ports for more devices. There are now true Thunderbolt Hubs which allow you to add more Thunderbolt devices without daisy-chaining. This opens up possibilities both for devices and displays.

A single Thunderbolt port on most Macs these days does support two displays – only one if it is an 8K monitor or using high refresh rates – and Thunderbolt Hubs provide support for this.

I haven't used one of these devices, but something like OWC's Thunderbolt 5 Hub is definitely on my radar for my next desk update. I have seen a video demonstrating three displays on the Thunderbolt 5 Hub. Still, I have not tried it, personally. OWC does also provide their own Apple Silicon External Display support article to attempt to clarify display support.

Other Articles

This is such a popular problem that while I've been writing this article, Macworld came out with an article of their own on this topic. They suggest primarily DisplayLink options, though, so take that as you will.

Conclusion

There are honestly too many things to keep in mind when trying to determine exactly how to connect a specific Mac to more than one external display. Much depends on your setup and use cases.

• How many displays do you want to connect?
• What resolution are those displays?
• Will you use the computer with the lid closed?
• How many cables are you willing to put up with?
• Are the trade-offs of software-based solutions acceptable?

There aren't any universal answers to these problems, other than saying, "Do your homework." I hope I've provided some sources and starting points to help you on your multi-monitor journey.