When summing up a circumstantial Crown case to a jury, prosecutors often draw an analogy to a rope.
Any one strand of the rope may not be strong enough to carry the burden of proving someone's guilt, but woven together the rope can carry the weight beyond a reasonable doubt.
The logic that underpins a circumstantial case is that the defendant is either guilty or is the victim of an implausible, unlikely series of coincidences.
The case against Scott Watson was circumstantial. And to carry on with the rope analogy, two strands of hair were by far the most damning evidence against him at his trial on charges of murdering Olivia Hope and Ben Smart.
Their disappearance from the Marlborough Sounds after a New Year's Eve party in 1997 and Scott Watson's convictions in 1999 for killing the two young friends have divided the country ever since.
Watson is still in prison and maintains his innocence, failing to overturn his convictions at the Court of Appeal or even convince the Privy Council to hear the case in London.
An application for the Royal Prerogative of Mercy in 2008 fell on deaf ears, after a review by Queen's Counsel Kristy McDonald advised it should be declined.
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Much of the 149-page report by the Wellington-based barrister focused on what former Furneaux Lodge staff, Guy Wallace and Roz McNeilly, now say about their evidence, which helped the Crown identify Scott Watson as the "mystery man" last seen with Ben Smart and Olivia Hope.
The QC concluded it did not meet the legal definition of "fresh evidence" for the case to be sent back to the Court of Appeal.
This was, essentially, because the evidence was available at trial.
Wallace said under cross-examination that Scott Watson - as shown in the Mina Cornelia photograph - could not have been the mystery man.
This allowed the defence team to vigorously challenge Wallace's initial identification of Watson, said McDonald, which they did.
In his interview with McDonald, Wallace went further, to say that, if asked in court now, he would categorically deny Watson was the mystery man.
This is called a "dock identification", but, again, McDonald was not convinced this changed the evidence Wallace gave at the trial.
And while Wallace's identification of Watson was important to the prosecution case, McDonald said there were other strands of circumstantial evidence the jury relied on to convict him.
The most "compelling" of these were two blonde hairs found on a blanket, with a tiger pattern, retrieved from Scott Watson's bunk in his yacht.
According to ESR scientists, DNA testing showed the hairs probably belonged to Olivia Hope.
While it's impossible to know what jurors consider important, the hairs were the only physical link between Watson to either Ben Smart or Olivia Hope.
"Compelling" is how Kristy McDonald described the evidence, in the context of a circumstantial case.
"It needs to be borne in mind that Mr Watson denied Ms Hope and Mr Smart were ever on his vessel," wrote McDonald.
"If the jury accepted the DNA evidence as reliable it must follow that, in the absence of a
credible alternative explanation Mr Watson could not have been telling the truth
about the two victims having boarded his vessel."
Doubt was cast over the source of the two hairs (labelled as exhibits YA69/12 and YA69/13) at Watson's 1999 trial.
About 400 strands of hair were taken from the blanket. No blonde hairs were found on the first examination in January 1998, taken from the Blade during the homicide investigation
A few months later, in March, after reference samples of hair believed to belong to Olivia were sent to the ESR laboratory, the tiger blanket strands were checked again.
They were examined on the same day, by the same scientist, on the same table as the reference samples from the Hope family residence.
There was also a cut in the reference sample bag holding her hair.
This time, two strands of blonde hair - one 15cm long, the other 25cm - were found in the blanket sample.
These circumstances raise the possibility of accidental contamination. But new scientific analysis commissioned by a Watson supporter, Brian McDonald, questions whether the hairs even belonged to Olivia Hope.
The original 22-page report written by forensic scientist Sean Doyle claims there were significant weaknesses in how ESR handled the hairs and identified the DNA.
"The hair and DNA evidence falls some way short of current standards and, in some respects, fell short of standards at the time."
He wrote a second report which said it was wrong for the scientists who gave evidence for the Crown to conclude there was "strong support" that the hairs belonged to Olivia Hope.
The test results were much weaker than suggested, in Doyle's opinion, to the point the two strands of hair could belong to a female relative of Olivia Hope.
If true, Doyle says this strengthens Watson's defence that the two hairs found on the tiger blanket were sourced from the reference samples.
This evidence formed the primary basis of a second application for the Royal Prerogative of Mercy lodged with the Ministry of Justice in November 2017.
A former High Court judge, Sir Graham Panckhurst QC, was instructed to review the second application, as well as the material considered by Kristy McDonald and her conclusions.
On reading Sir Graham's "comprehensive" advice, Justice Minister Andrew Little recommended to the Governor-General that Watson's application be granted.
Nearly 21 years after he was found guilty of murdering Ben Smart and Olivia Hope, Scott Watson's convictions will be referred back to the Court of Appeal for another hearing.
Watson will be hoping to join two other convicted killers, David Bain and Mark Lundy, in winning a second trial after long legal battles.
Bain was acquitted, Lundy convicted for a second time.
Watson's legal team are likely to again attack his identification as the "mystery man", the veracity of the secret "prison snitches" Watson supposedly confessed to, as well as sightings of the ketch dismissed by the Operation Tam investigation.
The prosecution case could be hanging by a thread.