adding memory to late 2012 iMac 27"

To get better performance, the memory with the same size must be installed horizontally, so Dual Channel will be turned on and the performance will be increased

You have to install in pairs horizontally, one next to the other one

The answer is that matched memory will be slightly faster but not enough for most users to see any difference.

And the important point here is that the memory has to be matched for there to be any affect at all, identical in every respect not just size. So unless you;re buying matched sticks just buying two sticks of the same size does not guarantee that they will be matched.

If when you got the machine the 2 sticks were in slots 1 & 3 then we'll assume Apple knew what it was doing when it installed the memory and just add your new sticks to the empty slots 2&4.

regards

Once you have it in and booted run About This Mac->More Info; Memory. It will show you a graphical representation of your memory slots.

This from my 2009 iMac.

I had the same question as mine came from the factory with 2 x 4 Gb memory installed in slots 1 and 3 also. No user manual yet for the late 2012 27" iMac 😟

On older macs I was used to placing memory side-by-side for matching pairs, but I too thought Apple knew what they were doing and expected us to install additional memory in the empty slots. So that's what I did with my 2 x 8 GB chips ($115 from OWC), and all seems to be working well. Here's what I see in system profiler:

I was uncertain about the matched pairs being in different banks, but after reading this thread relating to the 2011 iMac, I decided it was correct:

https://discussions.apple.com/thread/3082658?start=0&tstart=0

This is somewhat curious if Apple is now installing the RAM matching vertically? Until now, they have always installed the RAM matching it horizontally with the matching modules next to each other, as shown in your screenshot.