RAM

Mixing RAMs isn't supported, it may not work right and you can have blue screen errors on your machine. 

RAM is divided into Modules, Ranks (banks, rows and columns).

The difference between an 1Rx8 and 1Rx16 module is the layout of the memory. 1Rx8 means 1 rank with 8 bit wide chips, so to get a 64 but wide rank you need 8 chips on that rank. 1Rx16 means 1 rank with 16 bit wide chips, so a 64 bit wide rank uses 4 chips. Now, we get into memory controllers. The memory controllers on modem CPUs can control 8 bit and 16 bit chips up to a specific density, the total number of bits each chip can store.

All Core chips before the 5th Gen can control memory up to 4 Gbit density (exception, the very first gen Core i7 CPUs and Arrandale mobile CPUs, these go up to 2 Gbit.) This means a 4 GB module needs to be at least 1Rx8. An 8 GB module needs to be 2Rx8 on those CPUs. From the 5th Gen to the 7th Gen can control chips up to 8 Gbit density. So an 8 GB module can be 1Rx8 and a 16 GB module needs to be 2Rx8. From the 8th Gen onwards 16 Gbit chips are supported. On those CPUs a 16 GB module can be 1Rx8 and a 32 GB module needs to be 2Rx8.

CPUs before the 5th Gen can control a 4GB 2Rx16 (never seen one) module or a 2 GB 1Rx16. 5th-7th Gen control an 8 GB 2Rx16 (never seen one) or 4 GB 1Rx16. 8th Gen and later can control 16 GB 2Rx16 (never seen one) or 8 GB 1Rx16.