Twins often develop special bonds that look unbreakable from the outside, leading them to trust each other more than anyone else while growing and developing together throughout their lives.
For June and Jennifer Gibbons, this bond was so strong that they began isolating themselves from outsiders, eventually spiralling into a turmoil of pain and crime.
The pair became known as the "Silent Twins" for their refusal to speak to anyone except each other and their dolls.
The twins were born in a military hospital in Yemen in 1963 to parents Aubrey and Gloria, who were originally from Barbados.
Shortly after they were born, the family moved to the RAF base in Linton, Yorkshire, where the girls were raised.
Their parents became concerned when June and Jennifer failed to meet the language milestones expected at their age - and as time went on, teachers noted that the twins communicated freely with each other as well as their dolls, but would refuse to speak to outsiders.
Gradually, they began withdrawing from their family too, apparently forming an even deeper bond with each other which saw them communicating in a "secret language" no one else could understand.
When June and Jennifer were eight, the family moved to Devon for their dad's job before being transferred again to Pembrokeshire three years later.
The twins had a hard time at school as they were victims of bullying - during that time, they refused to read or write and also started mirroring each other's behaviour.
In 1974, medic John Rees visited the school and after administering a TB vaccination, he reported the girls to be unusually non-reactive, going so far as to describe their behaviour as "doll-like".
The girls were referred to several psychologists but no one was able to break their mysterious silence.
However, in February 1977, a speech therapist named Ann Treharne was able to record the twins speaking and worked out their "secret language" was a mixture of Barbadian slang and English, spoken very quickly.
They were then moved to the Eastgate Centre for Special Education - but the new school made little difference and they remained silent during therapy sessions.
In 1977, a decision was made to separate the twins to see if they would speak, with June being sent to St. David's Adolescent Unit.
But this did not help as June stopped moving altogether, on some occasions remaining in bed at the centre, apparently being unable to function without her twin by her side.
The sisters were eventually reunited at Eastgate and moved in between various institutions - but during this time, they retreated further into themselves.
Journalist and mental health campaigner, Marjorie Wallace, then became particularly interested in the twins and began getting to know them.
The author, who then wrote a book about Jennifer and June, described their relationship as "a sinister childhood game that got out of control".
She said that the twins had developed "rituals" between them where they would decide which sister would breathe first- and the other was not allowed to breathe until the first had taken a breath.
When the girls turned 16 they left school and returned home to Haverfordwest in Pembrokeshire, their extraordinary bond seemingly as strong as ever but now facing the added difficulties of adolescence too.
Despite not communicating verbally, they wrote many diaries, essays, poems, short stories and novels, that were later uncovered by the police.
One of June's books, Pepsi Cola Addict, about a student being seduced by a teacher, was self-published.
In one diary entry about the sibling's relationship Jennifer wrote: "We have become fatal enemies in each other's eyes.
"We feel the irritating deadly rays come out of our bodies, stinging each other's skin. I say to myself, can I get rid of my own shadow - impossible or not possible?
"Without my shadow, would I die? Without my shadow, would I gain life, be free or left to die? Without my shadow, which I identify with a face of misery, deception, murder."
By 1981, the girls discovered drinking and drugs, leading their lives to further derail.
In October of that year, they went on a five-week crime spree committing vandalism, burglary and even arson - getting caught red-handed trying to burn down Pembrokeshire Technical College.
The following year they pleaded guilty to 16 counts, but instead of jail time, the twins were Sectioned under the Mental Health Act and sent to Broadmoor Hospital.
During their 11 years of detention, the girls continued to write in their diaries, giving those around them a glimpse into their world. The conditions at the institution were unlike anywhere the twins had experienced before and they wrote about their longing to escape.
In 1993, with Wallace's advocacy, the decision was made to transfer the girls to a lower security facility closer to their family. But on March 9 as they got into the van to go to their new "home," staff noted Jennifer looked very weak and appeared unwell.
She was taken to the Princess of Wales Hospital in Bridgend, but sadly died at 6.30pm that same evening at the age of 29. Jennifer's postmortem revealed she had undiagnosed myocarditis, an inflammation of the heart.
After her tragic death, June remained at the lower security facility for a year, before finally being released and allowed to build a new life.
After getting to know the women relatively well, Wallace claimed that they had started to believe one of them had to die so the other could live.
She wrote that during an interview, Jennifer said she had decided to die so that June could live a normal life.
A poem written by June is engraved on Jennifer's headstone.
It reads: "We once were two.
"Through life be one.
"Rest in peace."
The twins' life has inspired theatre, songs and films, and raised important questions about mental health services in the UK.
