The DDR3-1066 DIMMs for the Mac Pro require a Thermal Sensor for full performance and compatibility. Many 3rd party Mac Pro DIMMs do not include this sensor. MemoryMasters memory always conforms 100% to the Apple spec, and these DIMMs include the Apple thermal sensor. The DDR3-1066 DIMMs for the Mac Pro will run 5-10% faster if they are installed in matched sets of three (tri-channel). You may add DIMMs in any combition, but performance will suffer slightly if the DIMMs are not matched 3x at a time. Additional Details ------------------------------ Package quantity: 1