Dual rank VS Single rank

what are the definitions of dual rank and and single rank memory?
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single rank is single a dimm with chips on 1 side and double rank means there are chips on both sides of the dimm
that's correct, also you can look on the sticker of the memory and there should be a 1R or 2R number.
DDR2 has limitations on the number of ranks per channel that can be supported, which is why you might need to spec out the ranking of your memory before trying to upgrade.
The "rank" of the chip is equivalent to the memory "Bank(s)" that are available of the mainboard. It is not the same as slots. Contrary to bob's statement, some memory sticks that have chips on both sides are single rank and some memory with chips on  one side are dual rank.
Mainboards can be varied in their number of banks per slot also. For example: one of my mainboard has two memory slots and four banks...it will accept up to two dual rank memory sticks..to occupy all four banks in the two memory slots. Another of my boards has four slots but only six banks avilable...thus to occupy all four slots with only six banks...I must use 4 single rank memory chips that occupy only four of the banks. If I use dual rank memory on this board, I am limited to occupying only three of the slots and all six banks of memory.
When buying memory...it is essential to know:
How many slots and how mank banks of memory are available on the mainboard... and
whether the memory is single or dual rank.
Everest Home Edition provides this information for an existing system or search the mainboard manual/mfgs website and the memory module specs.

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