THERE are 129 members of the Scottish Parliament, elected using what is known as the Additional Member System.

Scotland is divided into 73 constituencies, which each have one MSP. In addition, the country is divided into eight parliamentary regions, which are each represented by seven MSPs. It means 56 regional MSPs will be elected in addition to the 73 constituency members.

Everyone is given two votes. The first ballot paper, coloured lilac, is used to elect a constituency MSP. The constituency seats are decided using First Past the Post - meaning whichever candidate gains the most votes wins.

For this ballot, voters pick an individual, who will be representing a party unless they are standing as an independent. This is the same system that is used in General Elections to elect MPs to the UK Parliament.

The second ballot paper, which will be coloured peach, is used to elect the regional MSPs. With this ballot, voters are asked to select a party rather than an individual, although independent candidates may also stand for regions.

The regional seats are allocated under a system designed to make the overall result more proportional, following a calculation that takes the number of constituency seats a party has won in that region into account.

The number of votes cast in the regional ballot for each party or individual candidate is divided by the number of constituency seats they have gained, plus one.

It means that if a party has won none of the constituency seats in a region, its number of regional votes will be divided by one, therefore remaining the same. If a party has won six constituencies in that region (each region has eight, nine or ten constituencies within it), then its total number of regional votes would be divided by seven.

The first of the seven regional seats is allocated to whichever party has the most votes, after the first calculation is carried out.

To allocate the second to seventh additional seats the calculation is redone, but each time any additional seats gained are added into the calculation.

For example, if a party had won no constituencies in a region but found itself with the highest number of regional votes after the first calculation, it would be allocated the first regional seat but see its total vote divided by two for the second calculation. The process continues until all of the regional seats have been allocated.

Regional MSPs are selected from lists compiled by the parties.

The voting system has caused a fierce debate to erupt among supporters of pro-independence parties over how best to use the regional vote.

If the SNP do as well as predicted by opinion polls, it will take almost every constituency in Scotland. This would make it harder for the party to pick up list MSPs, as its regional vote will be divided so significantly. It has led to claims that a second vote for the SNP would be "wasted" and that those who want to see a maximum number of pro-independence MSPs would be wise to back a smaller party, such as the Greens, Rise or Solidarity.

The SNP has strongly urged its supporters to ignore the temptation to back another pro-independence party with the regional ballot, pointing out that it relied on list MSPs in 2011 to win its historic majority.

Professor John Curtice, the UK's top polling expert, has said: "Given the character of the polls – leaving aside the caveats about the accuracy of opinion polls on the list vote – some Yes voters may think it’s a waste of time voting for the SNP on the list vote...I’m simply pointing out there may be a tactical dilemma here, although it’s one with risks associated with it."