The Welsh Assembly is elected on the Additional Members System, where Wales is split into 5 regions. In each region are between 8 and 10 constituencies, all of which elect an AM. Then 4 additional AMs are elected on party lists using the d'Hondt system in each region to make the system more proportional.
At the May 1999 and May 2003 elections, dual candidacy was allowed, so someone could be a constituency candidate and be on their party's regional list. In theory, a constituency AM could lose their seat but eemain an AM by becoming a regional one, and conversely, a regional AM could stand in a constituency and win. This is still the rule for the Scottish Parliament.
The classic example is in the Scottish Parliament, with the Galloway & Upper Nithsdale constituency. At the 1999 election, the constituency was won by the Scottish National Party's Alasdair Morgan (the sitting MP). Also representing the constituency - as a regional member for Scotland South - was the Conservatives' Alex Fergusson.
At the 2003 election, the good people still had Morgan and Fergusson representing them. But something had changed. In the constituency, Fergusson had defeated Morgan by 99 votes, but Morgan remained a Member of the Scottish Parliament by virtue of his high placing on the SNP list in Scotland South.
The May 2007 election saw another battle between Morgan and Fergusson, with the latter increasing his majority (and becoming the Parliament's Presiding Officer), and the former continuing to represent Scotland South.
One reason for abolishing dual candidacy was the interesting result in Clwyd West at the 2003 election. The constituency was won by Labour's Alan Pugh, who, obviously enough, became an AM.
In second place was the Conservatives' Brynle Williams, followed by Plaid Cymru's Janet Ryder and then by the Liberal Democrats' Eleanor Burnham. All three of them became regional AMs for Wales North - indeed Ryder and Burnham were sitting regional AMs.
There was a slight advantage for Labour in abolishing dual candidacy as in both Wales and Scotland there was a tendency for it be Labour AMs/MSPs losing to sitting regional representatives who had built up a profile locally.
One thing to note, however, is that there are other ways for a politician to build up a profile. In South East England at the June 2009 election to the European Parliament one of our Members of the European Parliament was the Green Party's Caroline Lucas. Her work as an MEP since first being elected in June 1999 gave her a high profile which helped her to win Brighton Pavilion at the May 2010 general election.
And, for the House of Commons, being an AM or MSP can help build a profile that helps a politician become an MP. The Conservatives' Alun Cairns, who won Vale of Glamorgan from Labour at the 2010 election was a regional AM for Wales South West (interestingly, this region did not contain his new cnstituency, which is in Wales South Central). Another Conservative, the Minister for Scotland, David Mundell, won Dumfriesshire, Clydesdale & Tweeddale from Labour at the May 2005 general election while being a regional MSP for Scotland South.
The traffic isn't always towards Westminster though. At the 2007 election, the SNP leader, Alex Salmond, made his return to the Scottish Parliament after a 6 year absence by winning Gordon from the Liberal Democrats while still MP for Banff & Buchan, and then became First Minister - the post he still holds. The 2011 election saw Plaid Cymru's Simon Thomas become an AM for Wales Mid & West - he had been MP for Ceredigion from a by-election in February 2000 (caused by the resignation of Plaid Cymru's Cynog Dafis the previous month to concentrate on his duties as an AM for Wales Mid & West) until being defeated by the Liberal Democrats at the 2005 general election. 2011 also saw the Liberal Democrats' Willie Rennie - who now leads them in the Scottish Parliament - elected as a regional MSP for Scotland Mid & Fife, having been MP for Dunfermline & West Fife between winning it from Labour at a by-election in February 2006 and losing it to Labour at the 2010 election.
There should really be more traffic that way. Labour missed a trick at the 2011 election. Why not put the former Prime Minister and MP for Kirkcaldy & Cowdenbeath, Gordon Brown, top of the Scotland Mid & Fife list? Parachute former Chancellor of the Exchequer and MP for Edinburgh South West, Alistair Darling, in as Labour candidate for Edinburgh Pentlands (and high on the list in Lothian as an insurance policy), so he can lead Better Together as an MSP? Bung former Home Secretary and former MP for Airdrie & Shotts, John Reid, top of the Scotland Central list? (with former Scottish Secretary, Helen Liddell, second?)
How different would Holyrood be with Labour MSPs who have held high political office - in one case the highest political office?
Now, the Conservative/Liberal Democrat Government is considering abolishing the ban on dual candidacy, which would enable Wood to stand in both Rhondda and South Wales Central.
Rhondda itself has an interesting history. At the 1999 election it was actually won by Plaid Cymru, before losing it to Labour at the 2003 election. But it has an older history.
At the February 1974 general election, Rhondda was formed from two existing seats - Rhondda East and Rhondda West.
Rhondda West was solid Labour since it was created at the December 1918 general election. Its inaugural MP was William Abraham, who had been MP for the old Rhondda seat since the November/December 1885 general election when he was elected as a "Lib-Lab" candidate, i.e. notionally a Liberal but supported by the trade unions.
It is fair to say that Rhondda was solid Labour before the Labour party even existed.
Unlike today, when constituencies have long lists of candidates, this is an era of few candidates. Indeed, st the October 1931 general election the Conservatives and Liberals didn't contest the seat, so the sitting Labour MP, William John, was only challenged by the Communists. At the November 1935 and July 1945 general elections, he didn't face any challenger - indeed, in 1945 John was the only Welsh MP to be elected unopposed.
At the February 1950 general election, John retired, and the seat was won by Labour's Iorwerth Thomas. However, he died in December 1966, hence there would need to be a by-election.
By this stage, Plaid Cymru was making its presence felt - just as the SNP was north of the border. It had appeared on the scene at the May 1929 general election when it only contested Caernarvonshire. In May 1966, Megan Lloyd-George, who had been Labour MP for Carmarthen since winning it from the Liberals - the party her father once led - at a by-election in February 1957, died. Hence a by-election was held in July 1966 - the first since the March 1966 general election.
Plaid Cymru - like the UK Independence Party since the last general election - had had some by-election success, albeit not winning seats. Second place at Ogmore in June 1946 (albeit with no Conservative or Liberal candidates). Second place at Aberdare in December 1946 (pushing the Conservatives into third place) - something repeated in the same seat in October 1954.
Basically getting second place in seats Labour easily held. But Carmarthen changed all that. Plaid Cymru's leader, Gwynfor Evans, who had come third in October 1964 and again in 1966, won the by-election, but lost it back to Labour's Gwynoro Jones at the June 1970 general election. In February 1974 Jones held to it, with a majority of 3 votes over Evans. but Evans won it back in the October 1974 general election, before losing it to Labour in the May 1979 general election.
And in March 1967 it was Rhondda West's turn. Despite a 29% swing from Labour, and Plaid Cymru getting a higher share of the vote than in Carmarthen, Alec Jones held it for Labour.
The next Welsh by-election was Caerphilly in July 1968. Another 29% swing, a higher share of the vote than Rhondda West, but Plaid Cymru failed to defeat Labour.
That's Rhondda West. What about Rhondda East? Another fascinating seat.
Like its neighbour, Rhondda East was solid Labour during its existence. But it's when it comes to the runner-up that everything changes.
In 1929 the Communists contested the seat for the first time, getting a respectable third place behind the Liberals and pushing the Conservatives down to fourth place. In 1931 the Conservatives and Liberals didn't contest the seat, and - as in Rhondda West - Labour was only challenged by the Communists.
The by-election of March 1933 was won - predictably - by Labour, but the Communists' Arthur Horner (who would later run the National Union of Mineworkers) came less than 3,000 votes from becoming the sole Communist MP (at the 1929 general election Shapurji Saklatvala lost Battersea North to Labour).
From the 1935 to 1966 general elections - except for the Conservatives coming second at the October 1951 general election - the Communists were second in Rhondda East, until at the 1970 general election they were pushed into third place by Plaid Cymru.
And Wood becoming AM for Rhondda would be just another chapter in its unique political history.