1Rx8 means the memory chips are on one side of the RAM module and 2Rx8 means the memory chips are on both sides of the RAM module.Compatibility is generally not an issue in most cases. But you should go through the motherboard manufacture's guide of supported RAM modules just to make sure if they are compatible or not.R means the chips are all on one side, 2R means they're higher density on both sides. Some boards will not use high density with low density RAM chips or vise versa.