An interest rate is the amount charged by banks to borrowers for any loan, which is a percentage of the principal. It is an important monetary tool for central banks to control inflation and boost economic growth. The Hong Kong Monetary Authority maintains its official base rate in lockstep with the US Federal Reserve because the Hong Kong dollar has been pegged with the US currency since 1983. Hong Kong commercial banks can decide when to change their prime lending rate and deposit rate, which may differ in terms of timing and margin of change.