One curve shows exactly how far an economy is from everyone earning the same amount
In 1905, economist Max Lorenz plotted a simple graph: cumulative income earned against cumulative population, counted from poorest to richest. If income were distributed perfectly equally, the line would be a straight diagonal; in every real economy, it instead bows below that, since the poorest groups always hold a disproportionately small slice. That gap became the basis for the Gini coefficient, still the standard single number economists use to summarize inequality.