External display using and old iMac
Can I use an old iMac (20 inch, Mid 2007) as an external display with MacBook Air (13 inch, 2017) and if so, how?
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Please refer to Use your iMac as a display with target display mode. It has been archived, so its information may no longer be applicable.
There are hardware and software requirements: "Make sure that your iMac is using macOS High Sierra 10.13.6 or earlier. You can't use target display mode with later versions of macOS, or with Boot Camp and Windows."