Process–architecture–optimization is a development model for central processing units (CPUs) that Intel adopted in 2016. Under this three-phase (three-year) model, every microprocessor die shrink is followed by a microarchitecture change and then by one or more optimizations. It replaced the two-phase (two-year) tick–tock model that Intel adopted in 2006. The tick–tock model was no longer economically sustainable, according to Intel, because production of ever smaller dies becomes ever more costly.