what happens is that most ram runs in the "matching" slots. it's either 1 and 3 or 2 and 4. if you put ram in slots 1 and 2 or 3 and 4 then it disregards the second slot and only use either 1 or 3.
some mobos are picky and will only let you install 2 sticks in slots 1 and 3 when using only 2 sticks but will fail if installed in slots 2 and 4 only. slots 2 and 4 are reserved for when you buy 4 sticks of ram.
that's why you have color coded ram slots. if you have two then you must installed them on the matching color. be aware that if you were to get another pair and install it (assuming of course your mobo can handle them) then installed the matching pairs in the same color coded ram slots. by matching pairs I mean the 2 sticks that you bought together in the same package. never buy 2 or more sticks that come in different packages as these will not be matched to each other and there's always a chance that they may not play well together. however you can buy 2 sets of 2 to use all 4 ram slots, assuming of course your mobo can handle them. but even better is to buy all 4 sticks of ram in one package. they are guaranteed to play well with each other.
I hope you understand what I wrote as I also confused myself
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semp