Rates on residential property were based on the nominal rental value, reassessed periodically in revaluations. Rating system revaluations have taken place irregularly. Prior to 1999, revaluations were normally carried out every three years or longer. Revaluations took place in 1928/1929, 1934, 1956 (but based on 1939 values), 1963, and 1973. Some of these coins showed denominations which were no longer accurate due to earlier revaluations.