Quantitative tightening (QT) (or quantitative hardening) is a contractionary monetary policy applied by a central bank to decrease the amount of liquidity within the economy. The policy is the reverse of quantitative easing (QE) aimed to increase money supply in order to "stimulate" the economy. The QE policy was massively applied by leading central banks to counter the Great Recession that started in 2008. The prime rates were decreased to zero; some rates later went into the negative territory. For example, to fight with ultra-low inflation or deflation caused by the economic crisis, the European Central Bank, overseeing monetary policy for countries that use the euro, introduced negative rates in 2014. The central banks of Japan, Denmark, Sweden, and Switzerland also set negative rates.
The main goal of QT is to normalise (i.e. raise) interest rates in order to avoid increasing inflation as it becomes expensive to access money and reduces demand for goods and services in the economy. Like QE before it, QT has never been done before on a massive scale, and its consequences have yet to materialize and be studied. In 2018, the Fed began retiring some of the debt on their balance sheet, beginning quantitative tightening. In 2019, less than a year after initiating QT, central banks, including the Fed, ended quantitative tightening due to negative market conditions occurring soon after. 
An effect on asset pricesEdit
Whereas QE caused the substantial rise in asset prices over the past decade, QT may cause broadly offsetting effects in the opposite direction.
A living experiment - BitcoinEdit
Bitcoin is a payment network that approaches the direction of QT, while not quite a QT system. There is a fixed supply (~~21 million) of Bitcoin, with an inflationary supply schedule, albeit a decreasing supply, in its first one hundred years.
Research points that some Bitcoin are lost (e.g. due to lost keys), thus tilting Bitcoin closer to QT.
Proponents of QE and QE prone systems claim that deflationary spirals are linked to depressions. Decreasing prices in electronics easily disprove the notion that deflationary prices indicate economic depression.[unreliable fringe source?]
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