vacant memory slots
Which RAM Slots Should You Use? - How-To Geek
Yes, you can run two dual channel RAM configurations on a motherboard that supports four memory slots. This is known as quad-channel memory configuration.
memory - How do I clean a RAM slot? - Super User
The RAM slots are the slots where the random access memory (RAM) modules are inserted on the motherboard. The RAM is the memory that stores temporary data and instructions that the CPU needs to access quickly.
How do i do dual-channel with only 2 RAM slots?
Almost always the memory slots will be color-coded to indicate the memory channel, as shown in the picture below ... When installing memory in pairs make sure to install them into the same colored slot to use the dual-channel platform.