This is one of Wales's most volatile seats. In its previous form, as Carmarthen, it was the site Plaid Cymru's first-ever Westminster election victory, in the by-election of 1966.
Prior to that, it had been held for Labour since another by-election in 1957 by David Lloyd George's daughter, Megan.
It was then lost to Labour in 1970, whose candidate retained it by three votes in 1974 - the second smallest winning margin in UK General Elections since 1945.
It has remained in Labour hands since 1979 although the Conservatives came close in 1983. Plaid Cymru has posed an increasing threat over the last two general elections.
In the Assembly elections, Plaid won the seat, with a majority of 21.4%. This is Plaid's top target seat. It would take a 4.1% swing from Labout to topple Alan Williams.
Economically, the seat comprises rural farming areas to the west, which have been badly hit by recent agricultural crises, and traditional mining communities in the east.
Carmarthenshire is a strongly Welsh-speaking area, with 54% able to speak the language, and claiming the largest total number of Welsh speakers of any Welsh local authority.