THERE’S good news and bad news about the Single Transferable Vote system (STV) that will be used to elect Scotland’s local councillors on May 4.

The good news is that the actual process of voting is refreshingly straightforward. Voters simply need to rank the candidates in order of preference, using the number 1 for their first choice, 2 for their second, and so on. They can rank all of the candidates if they wish, but they don’t have to. So far, so easy.

But the bad news is that the procedure for counting the votes under STV is notorious for being almost comically difficult to explain.

It’s sometimes said that the system’s complexity isn’t really a problem. Essentially the argument is that voters don’t need to worry their pretty little heads over the counting process – all they need to do is vote, and leave the rest to the experts. But that’s a cop-out, because the way the votes are counted will obviously affect the way people want to use their votes, and in particular their decision over whether to use all of their preferences, or only some.

So how can a voter possibly get to grips with STV without poring over a long and impenetrable set of rules? Perhaps the best short-cut is to look at the principles that underpin the system. Once you understand what the system is trying to achieve, you can quickly develop an intuitive "feel" for how the rules will play out in different scenarios, without necessarily having to master what the specifics of the rules are.

Every electoral system has one or two basic objectives. For example, the first-past-the-post system we use for Westminster elections is favoured for producing strong majority governments and providing direct geographical representation. By contrast, STV is a proportional representation system, but that’s almost a side benefit – its primary aim is to empower the voter as much as humanly possible. It does that by being truly obsessional in its efforts to keep the number of wasted votes to an absolute minimum, even if that means taking into account the eleventh preference of a voter who otherwise need not have bothered turning up to the polling station.

In that sense, STV practically defines itself as being a solution to the problems of first-past-the-post, which invariably produces enormous quantities of votes that are wasted on unsuccessful candidates. And, for that matter, wasted on successful candidates as well, because the STV philosophy goes one step further and argues that a candidate’s margin of victory represents "surplus votes" that he or she doesn’t need, and that could be put to more productive use.

It may be helpful to reflect upon the name of the system at this point. STV gives you a "single" vote, which in the first instance is your first preference vote. When the votes are initially counted on election night, only the first preferences will be taken into consideration. In some wards, that will be the end of the story, because all of the available seats will be filled on the basis of the first preferences. However, just in case that doesn’t happen, your vote is also "transferable". This means your lower preferences are not (as many people wrongly assume) additional or multiple votes – they merely represent your instructions as to how your single vote should be used if your first choice candidate no longer needs it, either because they have already been safely elected, or because they have been eliminated to break the deadlock.

In theory, this transfer of votes should ensure that less than a quarter of votes will be wasted in every three-seat ward, and that less than a fifth of votes will be wasted in every four-seat ward. However, that will only be the case if every voter uses their full range of preferences.

If they don’t, there will be circumstances in which a ward’s final seat will be awarded by default to a candidate who has not reached the “quota” (the target number of votes at which any candidate is automatically deemed to be elected). The bigger the gap between the quota and the actual amount of votes received by the final winning candidate, the greater the number of voters who have missed a golden opportunity to influence the outcome on the final seat, because they failed to use enough of their lower preferences.

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So STV offers each voter considerable power, but it doesn’t force them to wield it. If they use all (or all but one) of their preferences, they wield all of the power available to them. If they use only some of their preferences, they squander some of that power, and leave other voters with a disproportionate influence on the final allocation of seats. This becomes a particular problem if certain categories of voters prove more likely than others to use all of their preferences.

AND there we arrive at the reason for fearing that the pro-independence movement may be sleepwalking into a minor crisis at the local elections. More than any other party, the Conservatives appear to have grasped the vital importance of advising their supporters to use most of their preferences. Tory election literature urges voters to rank the Conservative candidates highest, and then use the remaining preferences to rank all non-SNP candidates higher than all SNP candidates. Senior Tories have plainly understood that STV uniquely affords them the ability to pursue two separate objectives simultaneously – to maximise the number of Tory councillors, and to minimise the number of SNP councillors. There is no risk attached to chasing both goals, because a Tory supporter’s fourth preference for the LibDems, or sixth preference for Labour, cannot and will not be taken into account unless it gets to the point where all the Tory candidates have been elected or eliminated.

If anything, the opportunity open to the SNP is even greater. By urging their voters to use the full range of preferences, they could maximise their own number of councillors, and also the overall number of pro-independence councillors. They could minimise the number of Unionist councillors in general, and of Tory councillors in particular. But unfortunately they seem to be stuck in a mindset that is more appropriate for other electoral systems, and are declining to offer much advice beyond “vote SNP”.

As things stand, that’s bound to mean that the pro-Union forces will have an in-built advantage on May 4. There will be scenarios where the final seat in a ward is a straight fight between the SNP and Labour, and is decided in Labour’s favour by the lower preferences of Tory supporters. And yet the Tories may escape similar punishment where the final seat is a straight fight between themselves and the Greens, because the SNP election literature fails to advise people to rank other parties – even other pro-independence parties – ahead of the Tories.

NOW, let’s be clear – these are local elections, and you may well wish to opt out of the Tories’ game altogether. You can quite legitimately choose not to let your vote be dictated by the constitutional issue, in which case you should rank the candidates according to their own merits and their local policies. But, by the same token, there will be many people who hear the Tory call for these elections to be used to “send a message” about an independence referendum, and who passionately want that message to be the polar opposite of the one Theresa May hopes to receive.

If that sounds like you, here is your cut-out-and-keep guide to how to go about it.

1IDENTIFY exactly how many pro-independence candidates are standing in your ward. The number will vary significantly from place to place. There may be one, two or three SNP candidates. There may or may not be a Green candidate, or a Scottish Socialist Party candidate. Make sure you rank all of the pro-indy candidates with your highest preferences.

2 USE your next-highest preferences on any independent candidates, or candidates from fringe parties who are neutral on the constitutional question. It’s true that some independent candidates may be Tories or other Unionists in disguise, but it’s doubtful whether that really matters – the media will not interpret wins for those candidates as "Unionist wins".

3BY this point the only candidates you will not yet have ranked will be from the Unionist parties. If you want to minimise the number of Tory councillors, continue by giving your next-highest rankings to the LibDems and Labour. Remember there is no danger in doing this, because these rankings will not be taken into account unless only Unionist candidates are left in contention for a particular seat.

IF enough people follow all of these steps, the results could be startling. A recent Ipsos-Mori telephone poll of local council voting intentions put the Tories just two per cent ahead of Labour. If that’s even close to being accurate, a concerted drive by pro-indy voters to rank all other parties ahead of the Tories could conceivably consign Ruth Davidson to an unexpected third place finish in terms of seats, and deliver a massive psychological setback to Tory hopes of thwarting an independence referendum.