A tax on increases in the value of assets. CGT is usually only collected on the realization of gains by sale or bequest; to tax unrealized capital gains would require regular valuations of all assets. In the UK capital gains tax has been levied since 1965. Only gains over some minimum sum each year (£11,100 in tax year 2016–17) are liable to tax, and until recently it was levied only on proportional gains greater than the rise in the retail price index (RPI) since an asset was acquired. Certain forms of assets, including the main residence of any taxpayer, are exempt from UK capital gains tax.