When NAND flash memory was first used for general purpose storage in the earliest ancestors of modern SSDs, the memory cells were treated as simply binary, storing a single bit of data per cell by switching cells between one of two voltage states. Since then demand for higher capacity has pushed the industry to store more bits in each flash memory cell.

In the past year, the deployment of 64-layer 3D NAND flash has allowed almost all of the SSD industry to adopt three bit per cell TLC flash, which only a few short years ago was the cutting edge. Now, four bit per cell, also known as Quad-Level Cell (QLC) NAND flash, is the current frontier.