For 27-inch 2017 iMac, the apple support page says
Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:
PC4-2400 (19200)
Unbuffered
Nonparity
260-pin
2400MHz DDR4 SDRAM
I see on a lot of RAM on Amazon, then give the choice of either 2400MHz or 2666MHz. I'm...