Tuesday, April 6, 1999 Published at 17:58 GMT 18:58 UK
Click-through guide: The regional top-up system (1)
Additional Members are elected by adding together the total votes for each party (or individual standing on his or her own) in each constituency making up the region.
These totals are then divided by the number of constituency seats each party has won - plus one.
The party with the highest resulting total elects one Additional Member. That party's divisor is then increased by one (for its victory) and new figures calculated. Again, the party with the highest total wins a seat.
The process is then repeated until all four Additional Members are elected.
Here is an example of how the four Additional Members might be elected to the Welsh Assembly from the North Wales region.
It uses the voting figures from the 1997 general election and assumes for the sake of simplicity that people cast their party list ballots for the same party or individual as they did for their constituency vote.
For the sake of presentation, Others have been omitted from the following example, but it must be emphasised that the Additional Member System generally assists the election of small parties.
As you can see, Labour has polled the highest number of votes, but its divisor is very high (7 + 1 = 8) because it has won most of the constituency seats in the North Wales region.
The Conservatives have the highest total after dividing, so win the first Additional Member.