Gwent is a preserved county and a former local government county in southeast Wales. It was formed on 1 April 1974, under the Local Government Act 1972, and was named after the ancient Kingdom of Gwent. The authority was a successor to both the administrative county of Monmouthshire (with minor boundary changes) and the County Borough of Newport. (Both authorities which were considered to be legally part of England until the Act came into force, although they were considered jointly with Wales for certain purposes.)
Under the Local Government (Wales) Act 1994, Gwent was abolished on 1 April 1996. However, it remains one of the preserved counties of Wales for the ceremonial purposes of Lieutenancy and High Shrievalty, and its name also survives in various titles, e.g. Gwent Police, Royal Gwent Hospital, etc. "Gwent" is sometimes used as a synonym for the historic county of Monmouthshire — for example the Gwent Family History Society describes itself as "The key to roots in the historic county of Monmouthshire".
The former administrative county was divided into several districts: Blaenau Gwent, Islwyn, Monmouth, Newport and Torfaen. The successor unitary authorities are the Blaenau Gwent County Borough, Caerphilly County Borough (part of which came from Mid Glamorgan), Monmouthshire (which covers the eastern 60% of the historic county), City of Newport and Torfaen County Borough.
In 2003 the preserved county of Gwent expanded to include the whole of Caerphilly County Borough; the Gwent Police area had already been realigned to these boundaries in 1996. In 2007, the population of this enlarged area was estimated as 560,500, making it the most populous of the preserved counties of Wales.
The first three maps are provided by A Vision of Britain through Time
These maps were found on Wikimedia Commons
These maps of Glamorgan post-1974 were found on another site and are very useful for sorting out the up-to-date geography of the area