For centuries, the secrets of the Freemasons were known only to members.

But that’s about to change after the BBC were given unprecedented access to several Scottish lodges (the Freemason term for chapters or branches) to create a brand new documentary.

The society evolved out of Scotland after stonemasons working on the country’s public buildings in the 1500s banded together — in a similar way to modern trade unions — in a bid to protect the secrets and interests of their trade.

By 1599, William Schaw, who built castles and palaces for James VI of Scotland, had laid down rules for members to abide by, including: “They shall be true to one another and live charitably together as becometh sworn brethren and companions of the Craft.”

But it wasn’t until 1641 when soldier and diplomat Sir Robert Moray joined a stonemasons’ lodge that the movement really kicked off.

The Grand Lodge of Scotland’s curator, Bob Cooper, spoke with the BBC for the “Secrets Of The Masons” documentary, which will be aired in the UK on Tuesday.

Cooper said Freemasons were especially well known for their trademark handshake and rolled up pant legs.

“The handshake is a way of identifying one to another, especially when they had to move around Scotland looking for work,” Cooper said.

“If you went to a place where you weren’t known, you had to have some kind of proof that you were able to work on a construction site.”

He said rolled up pants were a symbolic gesture.

“What it means is that your skin touches the lodge so there is physical contact between you and the lodge,” he said.

“It’s just a very old and peculiar way of emphasizing the fact that you have entered an organization that you are never going to renege on.”

But perhaps the most intriguing aspect is the Freemason initiation ceremony — something so shrouded in mystery it has never been filmed, even to this day.

Current members are sworn to secrecy and details of what happens during a ceremony are murky.

However, ex-members who went on to leave the society have previously revealed some details online.

What is known is that the ceremony is based on a Bible story involving Hiram Abiff, who built the temple of Solomon but was attacked and killed after refusing to reveal the secrets of his building.

It is believed the biblical story is somehow re-enacted during a Freemason initiation ceremony, with the new recruit promising to never give away secrets of the society.

While no member would give away more details to the BBC, Brother Ewan Rutherford described his initiation ceremony as “one of the most frightening experiences I have ever had.”

Another reason why Freemasons are so endlessly fascinating to non-members is the belief that members of the group help each other to get ahead in life — a view that took hold thanks to the huge number of high-profile masons in the past and the rumor that masons in the police force would let fellow brothers off the hook for various offences.

It certainly worked for renowned poet and Freemason Robert Burns, whose secret society friends snapped up two-thirds of all copies of his first book in 1786.

But according to modern members, that’s no longer the case.

“I’ve heard it said that people only become Freemasons in order to get a leg up the greasy pole,” Cooper said.

“There are quite possibly occasions where people have got together within Freemasonry with the intent of doing that but that is not the purpose of the Freemasons.”

Another criticism of the organization is the fact that women have traditionally been barred from joining and participating.

However, Scotland’s Deputy Grand Master Ramsay McGhee told the BBC members benefited from gender segregation.

“We get criticised at times because we are men only but I firmly believe there are times when men need to be with men and there are times when women need to be with women,” he said.

“I think if we are tolerant of that fact then Freemasonry provides the ideal outlet for men.”

By Alexis Carey, News.com.au