"there is a list of compatible Memory released by the manufacturer" yes there is and that DRAM is compatible in that package, not mixed with other DRAM. I've gotten identical sets straight from manufacturers that won't play together, that's why they test sticks together befoer packaging i.e. so they can find 4 that will all play nice. You can pull them right off the assembly line and they might play, might not. Check with manufacturers I have seen one that will guarantee two packages of DRAM will play nice. If it was as easy as you try and make it sound manufacturers would simply sell single sticks, eliminate testing and make higher profits. I would guess that the bulk of what you work on is older DDR and DDR2 rigs which did mix easily, or low end systems running basic entry level DRAM. Also as mentioned - have you ever bothered to look in the forums, there are literally hundreds of threads of people trying to 'add' DRAM and problem after problem, often identical sets.