Extend vs. duplicate screen: what changes?

When you connect a second screen, you can usually extend your desktop or duplicate it. The choice changes whether you get more workspace or the same picture on both screens, and it can also affect how sharp or correctly sized that picture looks.

Use the Screens test to check which displays your computer sees and view each screen's resolution and scale. Then use your computer's display settings to choose a mode and check the result.

Check and change your screen setup

Open Screens and look at the list of connected displays. Check that both screens appear and note each one's resolution and scale. You can use the color test on a screen to confirm which display you are looking at.
Open your display settings. In Windows 11, go to Settings › System › Display. On a Mac, open System Settings › Displays.
Choose the mode you want. In Windows 11, select a display and open Multiple displays, then choose Extend these displays or Duplicate these displays. On a Mac, select a display and use the Use as options to extend or mirror the screen. The exact wording can vary by macOS version.
If you extend the desktop, arrange the screen icons in the settings to match the physical positions of your monitors. Move the pointer across the edge between them to check that it crosses in the right place.
Open a window and drag it between screens, or look for the same window on both. If the picture looks blurry or oddly sized, check the resolution and scale settings for each display.

What the result means

Both screens show the same picture

The displays are duplicated, also called mirrored. This is useful when you want to show the same presentation, video, or app on a monitor and a projector. Moving a window or changing the desktop on one screen changes what you see on the other as well. You do not gain extra desktop space.

Mirroring does not guarantee that the physical screens look exactly alike. They can have different brightness, color, sharpness, and size. A difference in shape or resolution can also cause the computer to scale the image or leave unused space on one display.

Each screen can show a different window

The desktop is extended. The second screen adds workspace, so you can keep a document on one display and a browser or call on the other. Drag a window to the edge of one screen to move it to the other. The pointer can also cross between them.

In an extended setup, each screen can use its own resolution and scale. The screens do not need to be the same size or model. The arrangement in display settings controls where the pointer crosses, not where the monitors physically sit.

One screen is blank, blurry, or cut off

The computer may be using the wrong display mode, an unsupported resolution, or an incorrect arrangement. A display can also be connected but not detected by the operating system. Check the display settings first, then check the cable, adapter, dock, and monitor input if the screen is still missing.

How to fix a confusing display setup

Confirm the mode on each display. In Windows 11, open Settings › System › Display, select a screen, and check Multiple displays. On a Mac, open System Settings › Displays, select a display, and check its Use as option. If you want independent workspaces, choose extend rather than mirror.
Arrange the screens to match your desk. In Windows 11, drag the display rectangles in Settings › System › Display so the left and right positions match your monitors. On a Mac, arrange the displays in System Settings › Displays. If the pointer seems to hit an invisible wall, the screen edges may not line up in the settings.
Set a suitable resolution and scale. In Windows 11, select the screen in Settings › System › Display and check Display resolution and Scale. On a Mac, open System Settings › Displays and choose a resolution option for the selected screen. A resolution lower than the display's native resolution can make text and images look soft.
Choose the main screen if windows open in the wrong place. In Windows 11, select the display in Settings › System › Display and turn on Make this my main display. On a Mac, arrange the displays in System Settings › Displays and set which one is the main display using the controls shown there. Some apps remember the screen where they were last opened.
Check the connection if a display is missing. Make sure the monitor is on and set to the correct input, then reconnect its cable. If you use a dock or adapter, try connecting the display directly to the computer if possible. Open the display settings again after reconnecting.
Restart after reconnecting the screens. If the settings show the correct mode but the picture does not update, disconnect and reconnect the display, then restart the computer. This can make the operating system detect the screens again.

Why duplicate screens can look different

Duplicate mode sends the same desktop content to multiple displays, but the screens still have their own physical properties. One might be a 1080p monitor and another a 4K television, for example. The computer has to fit the shared picture to both outputs. Depending on the computer and connection, it may choose a compatible resolution or scale the picture differently on each screen.

Extend mode gives each screen its own section of the desktop, so it is often a better choice when the displays have different sizes or resolutions. You can set a recommended resolution for each one without forcing them to show the same image.

The Screens test reports the displays and settings your browser can see. It does not change whether your desktop is extended or duplicated.