 Under the old system, by-elections were held only if two seats became vacant; however, the number of by-elections rose rapidly in the SMD system.Between 1947 and 1993, there were only eighteen incidences of by-election; whereas in the first two mixed-member elections, there were twelve by-elections.The Diet responded to this unanticipated consequence by holding by-elections on the same day twice a year for both upper and lower houses.