Defining the middle class is a perpetually popular parlor game because it's well-known that the foundation of widespread prosperity is a broad-based middle class and a sturdy ladder of social mobility that enables those below the middle class to work their way up to middle class security.

Here's an example of a typical trope on the subject: What Does It Take To Be Middle Class?

The topic is also a perennial favorite because the middle class is losing ground. By basic measures of income, it's slipped from 60% of the populace to 50%.

A strong case can be made that assessed by characteristics of middle class security and prosperity rather than income, the middle class has effectively shrunk to 10% of households as only the top 30% of households earn enough to afford what was within reach of the top 60% in decades past.

By definition, the top 20% cannot be "middle class." The top 20% is comprised of the upper-middle class, the wealthy and the super-wealthy (the 80% to 95% bracket, top 5% and the top 1%).

Attempting to define the middle class by income alone is futile due to regional differences in costs and purchasing power. $100,000 annual household income that will barely pay rent and necessities in a major metro city can stretch considerably further in smaller cities far from high-cost zones.