Freemasonry's roots are ancient but poorly documented. However, historians can trace the organisation almost continuously back to the 16th Century.
It evolved out of associations of stone masons who grouped together, possibly to control trade and help each other in times of hardship.
Links with stonework remain. Masons believe in a Supreme Being, which they call the Great Architect of the Universe and much symbolism relates to methods and tools. Colloquial terms for masonry include 'The Craft' and 'The Square'.
Britain's 350,000 masons are organised into about 8,500 lodges, affiliated to and regulated by the United Grand Lodge of England, based in Covent Garden.
Most lodges are open only to men, but there are also two organisations for women with their own lodges.
Members are invited to join, then undergo an initiation ceremony during which they swear to follow masonic principles and keep the masonic handshake secret.
The ceremony is based on the legend of Hiram Abiff, architect of King Solomon's temple, who was, according to the myth, murdered for refusing to betray masonic secrets.
Traditionally, these undertakings involved an oath under which new members acknowledged they would suffer a gruesome death in the event of betrayal. Grand Lodge says the oath was only symbolic and has now been dropped.
A good deal of lodge activity involves plays similar to the initiation rite. Members wear masonic regalia, including elaborately decorated lamb's wool aprons and jewels.
Freemasons traditionally kept a low profile because, they say, they wanted to prevent initiation rites being revealed to potential recruits. They generally rebutted any inquiries from outsiders.
However, in the last 10 years, under the leadership of Michael Higham, the Grand Secretary of Grand Lodge, the organisation has become more open, to dispel their "secret society" image.