First vote

For the first vote, for candidates in your local constituency, you would rank them all in your order of preference (ie, 1,2,3,4 instead of 'X').

To be elected, a candidate needs more than 50% of the votes. If none of them has, the least popular candidate is eliminated from the contest.

The votes that were cast for him or her are then re-allocated to the other candidates, based on second preferences (see graphic below). This process is repeated until one candidate wins.

This is how between 80 and 85% of the country's MPs would be elected under Lord Jenkins' proposal.

Second vote

Campaigners for proportional representation believe the popularity of parties among voters is not matched by the number of seats they win in Parliament.

So the second vote would be used to give any under-represented parties in a county or city a "top-up" MP.

These Top-Up MPs are nominated by their parties. Voters can either put their "X" against the party name, or - if they prefer - they can choose an individual candidate on the list.

The subsequent totals determine how much each party should be topped-up.

These Top-Up MPs would make up the remaining 15-20% of Parliament.

The Top-Up would mean that in an area which by tradition has overwhelmingly been represented by one party, other parties could now gain one, two or more MPs.

What if. . .?

Analysts estimate that if this system had been in place for Tony Blair's landslide election victory in 1997, Labour would have won a majority of between 77 and 115 seats.

Under the existing system, though, Mr Blair actually got a majority of 179.

The big winners under this scenario would be the Liberal Democrats, whose strength in Parliament would have doubled.