With the SNP dominating the constituency vote in opinion polls, a strong showing in the regional list vote is crucial for the opposition parties.

Here is a guide to Holyrood's second vote.

Why are there two votes for the Scottish Parliament?

A total of 129 MSPs are elected - 73 of these are constituency members, elected using the first-past-the-post system with voters choosing their preferred local candidate. The remaining 56 are regional seats, assigned using a system of proportional representation, with voters using their second vote to select a party or independent candidate.

The system is considered fairer than a purely first-past-the-post election because it makes the overall allocation of seats more proportional to the total number of votes received for each party.

How are regional members elected?

The constituencies are grouped into eight regions, with each region electing seven MSPs. In order to make the allocation of these seats proportional across the region, the number of first-past-the-post constituency MSPs already elected are taken into account when sharing them out.

The total number of regional votes received by a party or independent candidate is divided by the number of seats (constituency and regional) already gained in that region +1. The party with the highest number of votes after the formula is applied gains an additional seat. The calculation is repeated until all the additional seats have been awarded.

Why are the regional seats so important in the 2016 vote?

When it comes to the regional vote, the more seats you have already won, the harder it is to gain extra seats. With polls suggesting the SNP could clinch almost all the constituency seats, the party will find gaining regional seats tougher even if it snares a large percentage of the second vote, opening the door for its opponents.

How has this affected the campaign?

Firstly, rankings on the parties' regional lists were hotly contested. Those ranked in first place have the best chance because they are given the first regional seat awarded to their party, while those further down have little or no chance of winning a seat.

While the regional seats were once a secondary consideration for Labour, they are now viewed as key to its survival, with polls predicting the party could lose all of its constituency seats.

The Tories are reported to have concentrated most of their resources on the regional vote, while the Greens have focused on persuading SNP supporters to give them their second vote. The SNP has pressed its message of "Both votes SNP" as it attempts to consolidate its support.