Displays Have Separate Spaces on Mac: What It Does

If you run two monitors, one checkbox quietly decides how your entire desktop behaves: Displays have separate Spaces. It controls where the menu bar lives, what full screen does to your second display, and whether a window can straddle both. Here's what each mode actually means and how to switch — it needs a logout.

What ON means (the default)

With Displays have separate Spaces on, macOS treats each monitor as its own world:

• Every display gets its own menu bar — the active one is fully opaque, the others dimmed.
• Each display has its own set of Spaces. You can flip Desktop 2 to Desktop 3 on your left monitor while the right one stays put.
• Full screen is per-display. Taking a video full screen on one monitor leaves the other completely usable.
• The Dock sits on whichever display you summon it on (push the pointer to the Dock edge of a screen).

For most people this is the better mode, which is why it's been the default on every recent macOS — Ventura, Sonoma, and Sequoia alike.

What OFF means

Turn it off and your monitors merge into one continuous Space:

• One menu bar, on the primary display only.
• Windows can span the seam — half on one monitor, half on the other. This is the feature people turn it off for: a single giant timeline, spreadsheet, or DAW session stretched across both screens.
• Full screen captures everything: one app goes full screen and the other display turns to blank linen. This is the cost people forget.
• Switching Spaces switches both displays at once — a Space is now a pair of screens.

How to change it (logout required)

1. Open System Settings → Desktop & Dock.
2. Scroll to the Mission Control section.
3. Toggle Displays have separate Spaces.
4. Log out and back in (Apple menu → Log Out) — the change does not apply until you do.

Terminal equivalent — note the inverted name: spans-displays true means separate Spaces off:

# turn Displays have separate Spaces OFF (one Space spans all displays)
defaults write com.apple.spaces spans-displays -bool true

# undo: back to separate Spaces per display (the default)
defaults write com.apple.spaces spans-displays -bool false

# then log out and back in either way

Which should you use?

Keep it on if you use full screen apps, Spaces, or want independent monitors — that is, almost always. Turn it off for video walls, trading setups, wide creative timelines, or presentations where one canvas must cross the bezel. If you're not sure, you're an "on" person; the people who need "off" already know the seam-straddling feeling they're chasing.

What to expect the moment you switch

The logout isn't a formality — macOS rebuilds your whole display arrangement around the new model. Coming back with separate Spaces off, the per-display desktops you'd accumulated collapse into one shared row and windows pile onto the primary screen waiting to be redistributed. Budget a few minutes to put things back; nothing is lost, just re-homed.

Day to day in the off mode: the single menu bar lives on whichever display is primary — change that in System Settings → Displays by dragging the white menu-bar strip between screens in the arrangement diagram — the Dock follows the primary display too, and Mission Control shows one combined picture instead of a per-display overview. Switching back restores per-display menu bars after the next logout.

A testing tip: give the flipped setting a full working day, not five minutes. Spanning mode feels wrong for the first hour — full screen blanking a monitor is genuinely jarring — and only earns its keep once you've actually stretched a timeline across the seam. If you never do, that's your answer: go back to separate Spaces.

Related multi-monitor fix

If windows keep launching on the wrong screen after you change this setting, that's its own solvable problem: stop windows opening on the wrong display.