RAM slot 1-3 or 2-4? (dual channel Skylake DDR4)

CPUzX

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Jun 12, 2015
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My motherboard is an MSI Z170A Gaming Pro, my CPU is an i7 6700K, and my RAM is HyperX Black Fury 2x8 DDR4 2666MHz sticks, and I currently have the two sticks in slots 1&3 as 2&4 didn't boot. The one thing I'm a little on edge about is that in the motherboard manual, it states to put the RAM in slot 2 first, followed by a picture with two RAM sticks in slots 2&4. My PC didn't boot when I put mine in 2&4, but it does in 1&3. Does having the RAM sticks in 1&3 work on dual channel, the same as if I were to have the RAM in 2&4?

Also, would this RAM be automatically overclocked to 2666MHz automatically, as I've noticed in the bios, or do I have to activate XMP mode?
(I think it shows up as 1333mhz in CPU-Z). Thanks for the help!