In Scotland there are eight voting regions. In each region there are between eight and ten constituencies.
Voting takes the form of two votes. The first is a First Past the Post constituency vote and is carried out independently in each of the eight regions.
The total number of constituencies across all the regions is 73. There are an additional 56 regional list seats that are allocated in the second vote giving a total of 129 seats. Due to the allocation of these additional members to parliament as will be described below, it is very difficult to achieve an overall majority.
The D'Hondt proportional voting system is based on a number of voting rounds whereby each party is awarded additional partiamental members. Prior to this, the First Past the Post constituency votes are counted, and in each round, each party's votes are biased by being divided by the number of seats already won plus one. This addition of one is done because otherwise division by zero would occur where a party has not won any seats.
This process is repeated for each round until all the seats have been allocated. After each round, the party winning on that round has it's divisor incremented. Thus the party with the most votes does not necessarily get the most seats, especially if it has done well in the preceding constituency vote. In this case such a party would be at a disadvantage as it's starting divisor would be high.
The eight regions each have an additional seven 'seats' available to allocate, so there are seven D'Hondt rounds.
Sound complicated? Maybe not, but it is confusing to voters who don't know how it works. Many voters, spurred on by the leading independence-supporting party, the Scottish National Party (SNP), waste their list vote by giving it to them. If the SNP do very well in the constituency vote, it's very likely that they may not get any regional list seats because of the constituency seats they have already won. It would have been better to give the second vote to a different independence-supporting party that they could go into a coalition with and achieve independence for Scotland. The following might be helpful in explaining how the D'Hondt part works.
The alternative data is a scenario where the SNP voters would have
given their regional list vote to:
1. Alba/Green on a 50/50 basis, 2. All SNP votes to Alba, 3. All SNP votes to Green party.