Attached to many Lodges are what are referred to as Royal Arch "Chapters". 

Attached to Sidcup Lodge is St. Bride Chapter which originally met in London but now meets in Kent.

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Becoming a Royal Arch Mason is the forth and final step in Pure Antient Masonry as defined in a act of union in 1813.

 

Chapter implies being part of a story so the Royal Arch ceremony sees Masons follow clues to complete the journey of the 3rd degree. It is a spectacular ceremony that is open to members 4 weeks after becoming a Master Mason.