The men's underwear index (MUI) is an economic index that can supposedly detect the beginnings of a recovery during an economic slump.
The premise is that men's underwear are a necessity in normal economic times and sales remain stable.
During a severe downturn, demand for these goods changes as new purchases are deferred.
[1] Hence, men's purchasing habits for underwear (and that of their spouses on their behalf) is thought to be a good indicator of discretionary spending for consumption at large especially during turnaround periods.
This indicator is noted for being followed by former Federal Reserve Chairman Alan Greenspan.