How it works
One app, two Macs, one cable.
No account, no server, no router. Everything below happens between the two machines on your desk.
The short version
You install the same app on both Macs and connect them with a Thunderbolt cable. On the Mac you want to use, you click Connect. A moment later the other Mac is showing a second desktop, and macOS treats it like any monitor you would plug in — you drag windows onto it, arrange it in Displays, and forget it is a computer.
Both roles live in one app. There is no separate "server" download and no separate "client" download. Whichever Mac clicks Connect becomes the sending one for that session; the other becomes the screen.
What each Mac does
The Mac that sends
This is the Mac you keep working on. The app asks macOS for an additional desktop — the same kind of extra space you get from plugging in a monitor — and sends the picture of that desktop down the cable. Your built-in screen keeps working exactly as before; nothing about your main display changes.
This is the Mac that needs a license. The trial covers it for seven days.
The Mac that shows
This one just displays what arrives. The app fills its screen with the incoming picture and gets out of the way. Its own desktop is still there, underneath — press Esc on that Mac (or stop the session from the sending Mac) and you are back on it immediately.
This Mac is never licensed. Being a screen is free, forever, on as many Macs as you like. That is deliberate: it is the only way "use the old iMac in the corner" can be a five-minute decision instead of a purchase.
What the Thunderbolt cable is doing
When you connect two Macs with a Thunderbolt cable, macOS itself builds a small private network between them, called the Thunderbolt Bridge. It is a standard macOS feature, it appears in System Settings under Network, and it exists whether or not you use DisplayBridge. It has two properties that matter here:
- It is fast. Far faster than Wi-Fi or Gigabit Ethernet — which is the entire reason this app can send a picture without squeezing it first.
- It is private. The link goes cable-end to cable-end. It does not pass through your router, your ISP, or anything else. There is nowhere else for the picture to go.
You do not have to configure any of it. The two Macs find each other on that link on their own and appear in each other's list.
Why it has to be a 40 Gb/s cable
A full 5K picture at 60 frames per second is roughly 10.6 gigabits per second of raw image. Wi-Fi and Gigabit Ethernet are one to two orders of magnitude short of that. Anything wireless therefore has to throw most of the picture away and rebuild an approximation at the other end — which is exactly why every wireless option looks soft on text and smears during motion.
We would rather ship one path that always works than four that sometimes do. So: cable only, and we say so on the first page rather than in a footnote.
What macOS sees
An ordinary extra display. Concretely, that means:
- It shows up in System Settings › Displays, where you arrange it next to your other screens, set its position, and pick a resolution.
- Windows, Mission Control, full-screen apps and Spaces all behave the way they do on any monitor.
- Text is rendered at Retina sharpness, not scaled up from something smaller.
- It disappears cleanly when you stop the session. Your windows come back to your main screen the way they do when you unplug a monitor.
Sound follows the picture, and the brightness and volume of the receiving Mac can be driven from the session card on the sending one, so it behaves like a monitor there too.
The permissions macOS asks for
Two, and it is worth knowing which is which — they are asked for different reasons and on different machines.
Screen Recording — on the sending Mac only
macOS treats "read the pixels of a desktop" as a privileged operation, no matter whose desktop it is. Sending a picture of a screen is exactly that operation, so macOS requires the permission. The app asks once, and shows a card with a direct link to the right settings pane if it is missing.
The Mac that only shows the picture never needs it.
Local Network — on both Macs
This is what lets the two Macs see each other on the Thunderbolt link. Without it they cannot find each other and the receiver list stays empty. macOS asks the first time you open the app.
That is the complete list. No kernel extension, no admin password, no login item, no background service left running after you quit.
Where your screen goes: nowhere
The picture travels from one Mac to the other over the cable and stops there. It does not reach our servers — we do not run any for this — or your router, or the internet. There is no cloud stage, no relay, no account that the stream is attached to. We could not see your screen if we wanted to.
Two consequences worth spelling out:
- It works offline. Once your license is activated, streaming needs no internet connection at all. The app re-checks the license occasionally and is generous about not being able to — an internet outage never interrupts a running session.
- There is nothing to sign into. No account, no sign-up, no password. A license key from your purchase email is the whole of it.
The one thing the app does send is an anonymous signal that it launched and that a session started, plus the app and macOS version — no identifier that follows you, nothing about your screen or your files. It answers two questions for us: roughly how many people use DisplayBridge, and when it is safe to drop support for an old macOS. The details are on the privacy page.
What it deliberately is not
- Not a remote-control tool. The keyboard and mouse of the receiving Mac do not drive the sending one. It is a display, not a session takeover.
- Not wireless, and it never will be — see above.
- Not a way to use an iPad. That is Sidecar, and it is built into macOS already.
- Not a screen-sharing or meeting tool. Nothing is streamed to anyone, anywhere.
Next
The setup guide walks through it step by step. The comparison page puts it beside Sidecar, AirPlay, Luna Display and the rest. The FAQ has the rest of the questions people actually ask.