Money is used as a medium of exchange, a unit of account, and as a ready store of value. Its different functions are associated with different empirical measures of the money supply. There is no single "correct" measure of the money supply. Instead, there are several measures, classified along a spectrum or continuum between narrow and broad "monetary aggregates". Narrow measures include only the most liquid assets, the ones most easily used to spend (currency, checkable deposits). Broader measures add less liquid types of assets (certificates of deposit, etc.).