In the study of economics and finance, these labels serve as categories of the money supply, ranked by their liquidity (the ease with which they can be converted into cash). M0 is the narrowest form of money, consisting of physical coins and notes in circulation plus bank reserves. As you move toward M4, the categories include increasingly less liquid assets. M1 covers currency and checking accounts. M2 adds savings accounts and certificates of deposit (CDs). M3 includes institutional assets and larger deposits. M4 is the widest classification, acting as the most comprehensive measure of money available in an economy, helping central banks monitor inflationary pressures and financial stability.