These changes were reflected in the following redistribution of parliamentary seats which did not come into effect until the 1983 general election, resulting in a reduction in the county's representation from 16 to 7 MPs.
The table below shows the number of MPs representing Durham at each major redistribution of seats affecting the county.
[1] The Great Reform Act 1832 radically changed the representation of the House of Commons, with the County being divided into the Northern and Southern Divisions, both returning 2 MPs.
[5] Under the Representation of the People Act 1918, the number of constituencies in Durham was increased further, from 16 to 18, which resulted in significant boundary changes.
[7] The County Boroughs of Darlington, Gateshead, South Shields, Sunderland and West Hartlepool had all expanded since 1918 and the parliamentary boundaries were adjusted accordingly.
[10] The constituency of Sedgefield was abolished and its contents distributed as follows: Other changes included: Shortly after the Second Periodic Review came into effect, the county was subject to a major reconfiguration under the terms of the Local Government Act 1972.
[13][14] For the 2023 Periodic Review of Westminster constituencies, which redrew the constituency map ahead of the 2024 United Kingdom general election, the Boundary Commission for England opted to combine the unitary authority of Durham with the Tyne and Wear metropolitan boroughs of Gateshead, South Tyneside and Sunderland as a sub-region of the North East Region.
The table below shows which constituencies represented major communities within the current county (including Darlington) from 1885 onwards.