From https://support.apple.com/en-us/HT201255

Startup key combinations for Mac

Some Mac features are available by holding down certain keys during startup.

Press and hold the keys immediately after you turn on your Mac and hear the startup sound. Keep holding until the described behavior occurs. These combinations work with Intel-based Mac computers.



Hold during startupDescription
Shift ⇧Start up in Safe Mode.
Option ⌥Start up to Startup Manager.
CStart up from a bootable CD, DVD, or USB thumb drive (such as OS X install media).
DStart up to either Apple Hardware Test or Apple Diagnostics, depending on which Mac you're using.
Option-DStart up to either Apple Hardware Test or Apple Diagnostics over the Internet.
NStart up from a compatible NetBoot server.
Option-NStart up from a NetBoot server using the default boot image.
Command (⌘)-RStart up from OS X Recovery.
Command-Option-RStart up from OS X Recovery over the Internet.
Command-Option-P-RReset NVRAM. Release the keys after you hear the startup sound again.
Command-SStart up in single-user mode.
TStart up in target disk mode.
XStart up from an OS X startup volume when the Mac would otherwise start up from a non-OS X startup volume.
Command-VStart up in verbose mode.
Eject (⏏), F12, mouse button, or trackpad buttonEject removable media, such as an optical disc.



Other Mac keyboard shortcuts are available after your Mac has started up.


-RBB