Diet
The Diet (国会, kokkai) is the national legislature of Japan.
House of Representatives
the Shuugin (衆議院, House of Representatives) is the lower legislative house.
The lower house can override a vote of the upper house with a two-thirds majority.
At formation the house had 466 seats elected across 130 multi-member districts. Each district had N representatives roughly in proportion to their population. Each voter had one non-transferable vote. The top N candidates in a district received a seat in the house. A seat was added in 1955 for the Amami Islands, which the United States repatriated on Christmas Day 1953. 19 seats were added in 1967. 5 seats were added in 1972. 20 seats were added in 1976. 1 seat was added in 1986. 1 seat was removed in 1993.
The 1994 electoral reform transformed the lower house into a parallel system. 200 seats were elected across 11 regional blocks. Each block had N representatives roughly in proportion to their population. Within each block, seats were allocated based on a party list vote. Meanwhile, 300 seats were elected directly in single-member districts. In sum, the 511 seats were reduced to 500. In 2000, 20 seats were removed. In 2014, 5 seats were removed. In 2017, 10 seats were removed. the current house has 465 seats.
House of Councillors
The Sangiin (参議院, House of Councillors) is the upper legislative house.
At formation the house had 250 seats in staggered 6-year terms; half are allocated for election every 3 years. In 1970, two seats were added for Okinawa. Malapportionment reforms passed in 2000 saw 10 seats stripped from the house in phases by 2005. Further reforms passed in 2018 saw 6 seats added in phases by 2022. The current house has 248 seats.
150 seats are elected from 47 districts which have unequal representation. In any districts with just one contested seat, this system is equivalent to first-past-the-post. 6 seats were stripped from this amount in 2000, while 2 were added in 2018.
100 seats were elected from a national block (全国区), in staggered amounts of 50 per election. In 1983 this was replaced with a 100-seat proportional district (比例区) based on party lists. The aforementioned malapportionment reforms saw two seats were stripped away in the 2001 and 2004 elections each. Two seats were added in 2019 and 2022 each.
In 2005, Koizumi established a precedent for dissolving the House of Representatives after a vote of no confidence in the House of Councillors.