Use your iMac as a display
You can view the desktop of your Mac on the display of some iMac models using Target Display Mode. In some cases, you can also use Target Display Mode to play the sound from your Mac (called the primary Mac) on the speakers of the external iMac. For example, if you have a MacBook Pro you could use an iMac as the display and for playing audio.
Note: Target Display Mode isn’t supported on iMac models with Retina display. Only iMac (27-inch, Late 2009), iMac (27-inch, Mid 2010), and iMac (Mid 2011 to Mid 2014) support Target Display Mode.
Use an iMac with Thunderbolt as an external display
If you have an iMac with Thunderbolt, any other Mac with Thunderbolt can use it as a display. The iMac will play both the video and audio from the other Mac.
Connect the Thunderbolt cable to the Thunderbolt ports
on each Mac.Make sure both Mac computers are turned on and awake.
Press Command-F2 on the keyboard of the external display iMac.
If you want the iMac to play audio as well as video from the primary Mac, go to the primary Mac, choose Apple menu
> System Preferences, click Sound, then click Output. Select the Mac in the list of devices.
Use a 27-inch iMac with a Mini DisplayPort as an external display
If you have a 27-inch iMac with a Mini DisplayPort 
, any other Mac with a Mini DisplayPort or Thunderbolt port can use it as a display. Make sure you have a Mini DisplayPort to Mini DisplayPort cable.
If the primary Mac has a Thunderbolt port, it can play audio and video on the iMac. If the primary Mac has a Mini DisplayPort, it may be able to play its audio.
To see if it can, go to the primary Mac, choose Apple menu > About This Mac, click System Report, then select Audio in the Hardware category. A list of your computer’s audio devices appears. If HDMI Output or HDMI/DisplayPort Output is listed, your computer’s Mini DisplayPort can send audio. Make sure you use a Mini DisplayPort cable that carries both audio and video.
Connect the Mini DisplayPort cable to the ports on each Mac.
Make sure both computers are turned on and awake.
Press Command-F2 on the keyboard of the external display iMac.
If you want the iMac to play audio as well as video from the primary Mac, choose Apple menu > System Preferences, click Sound, then click Output.
Select the iMac in the list of devices.
Turn Target Display Mode off
You can easily turn Target Display Mode off without disconnecting the computers.
Press Command-F2 on the keyboard of the external display iMac.
When Target Display Mode is off, the external display iMac displays its own open apps.
Learn what happens when the primary Mac goes to sleep and wakes up
The external display iMac behaves differently depending on how the primary Mac goes to sleep.
If you put the primary Mac to sleep: After you put your Mac to sleep, the external display iMac goes dark for a few seconds and then displays the windows for its own open apps. If it doesn’t, press Command-F2 on its keyboard.
If the primary Mac goes to sleep because it was idle: The external display iMac goes dark and stays dark. If you want the external display iMac to display the windows for its own apps, press Command-F2 on its keyboard.
If the external display iMac doesn’t show the right desktop after it wakes up: When the primary Mac wakes up, the external display iMac displays the windows for the open apps on the primary Mac. If it doesn’t, press Command-F2 on the keyboard of the external display iMac.
Set options for the external display iMac
You set the options for an external display the same way you do for any other connected display.
After you connect the computers and turn on Target Display Mode, the external display iMac acts like any other external display. Apps that were open on the iMac remain open, even though you can’t see their windows.