Thanks. That Apple page got it.
For interested readers like me, with a Windows partition and a third-party keyboard plugged into a USB port, the procedure is this:
Type the following: sudo nvram boot-args="x"
Restart or shut down the Mac
As it starts, hold option key, then choose your Mac OS partition
While it loads, immediately press and hold the shift key
After the desktop comes up, to verify that you are in Safe Mode, go to the Apple icon in the upper left corner of your screen.
Go to About this Mac, then System Report
Scroll down to Software
Next to software, it will say Boot Mode:
Should read Safe rather than Normal
To get out of safe mode, open Terminal again
Type the following: sudo nrvam boot-args="" (no space between the quotation marks)
Quit Terminal and restart the computer
OS X 10.11.6
iMac 21.5", Mid 2011
2.8 GHz Intel Core i7, 24 GB
AMD Radeon HD 6770M
Using Apple computers since 1980