Credit for graphic and most of the text: Colin Dunn aka Indyposterboy

The voting system for Scottish Parliament elections was introduced by the UK Labour government in 1998. Its structure makes it difficult for any single party, like the Scottish National Party (SNP), to gain an outright majority. This is achieved through a mixed system that combines two types of votes, aiming for more proportional representation overall.

Each voter gets two votes on election day:

Constituency Vote: This is like the UK's Westminster "First Past the Post" (FPTP) system. Scotland is divided into 73 constituencies, and the candidate with the most votes in each wins a seat. (Total: 73 seats.)

Regional (List) Vote: This uses the "Additional Member System" (AMS). Scotland is divided into 8 regions, each electing 7 additional Members of the Scottish Parliament (MSPs) from party lists. Parties can choose to stand only for list seats if they want. (Total: 56 seats.)

The key feature: The system adjusts for constituency wins to make the overall result more proportional. If a party wins many constituency seats, their list votes are "devalued" (divided by a higher number), making it harder to win extra list seats. Conversely, parties with fewer constituency wins get more value from their list votes.

In the 2016 election:
The SNP won 59 out of 73 constituency seats.
The Conservatives and Labour together won only 10 constituency seats.
On the list vote: SNP got about 956,000 votes, while Conservatives and Labour combined got roughly 4,000 more (around 960,000 total).

Result: SNP's list votes were heavily devalued due to their constituency success, earning them only 4 list seats. Conservatives and Labour, with fewer constituencies, got 45 list seats from their similar vote total.
This shows how the system "balances" strong constituency performers by limiting their list gains.

If the SNP continues to lead in constituency polls, a strategic way to boost pro-indy MSPs and reduce unionist seats is:

Vote SNP on the Constituency Ballot (SNP1) to secure those seats.
Vote Scottish Greens on the Regional List Ballot (SG2), as they are typically well-positioned in all regions to win list seats without the devaluation penalty.

Exception: In rare cases where SNP and Scottish Greens are competing closely in a constituency, vote for the leading pro-indy candidate there (e.g., Leader1/SG2).

This tactic leverages the system's design to favour parties with fewer constituency wins on the list vote.

Annex

The list seats are allocated using the D’Hondt method, which acts like a "handicap" to promote balance. The formula for each party's score in each round is:

List Votes ÷ (Constituency Seats Won + 1)
Parties start with their divisor as (constituency seats + 1).
No constituency seats? Divide by 1 (full vote value).
Many constituency seats? Divide by a large number (votes heavily discounted).
The highest-scoring party wins a list seat, then their divisor increases by 1, and the process repeats until all seats are filled.

Think of it as a two-round competition:
Round 1 (Constituencies): Parties win "trophies" based on direct votes.
Round 2 (List): Bonus seats are awarded to even things out. Parties with lots of trophies get a big handicap (higher divisor), so their list votes count less. Parties with few or no trophies get little handicap, making their votes more effective.

Worked Example
Imagine three parties in a region allocating list seats:
 
 
 
 

First List Seat: Goes to Party C (highest score: 30,000).
Update: Party C now has 1 seat total, so next divisor = 2.
Recalculate scores: Party A = 9,090; Party B = 20,000; Party C = 30,000 ÷ 2 = 15,000.
Second List Seat: Goes to Party B (now highest: 20,000).
And so on, repeating until all seats are allocated.

This is why, in 2016, the SNP's 956,000 list votes only yielded 4 seats (high divisor from 59 constituencies), while Conservatives and Labour's similar combined votes got 45 seats (low divisors).

To summarise:The voting system introduced by the UK Labour government in 1998 makes it difficult for any single party, like the SNP, to gain an outright majority. The better the SNP does by winning constituency seats the higher the 'penalty' they receive and the less 'worth' the list seats become. Unless a list vote candidate is extremely likely (guaranteed almost) to win a list seat, there is precious little point in wasting a vote for the sake of it. 

Exception: In rare cases where SNP and Scottish Greens are competing closely in a constituency, vote for the leading pro-indy candidate there (e.g., Leader1/SG2).