Some extracts from Barker about making the purple velvet for the Royal Robe of State for Queen Elizabeth II.

By long tradition an uncrowned monarch does not wear the royal purple.
So, the Queen drove to the Abbey in her robe of crimson velvet.
The silk for the purple velvet had been spun by a special strain of silkworms.
At various different places, the silk was spun, made into yarn, dyed, then woven into lengths 20 yards long by 21 inches wide
"on a handloom indentical with those used 100 years ago."
The fabric was sent to "the unpretentious shop of Ede & Ravenscroft" where it was cut to shape before passing to the Royal School of Needlework.
The embroidery depicted olive leaves and ripening corn - to symbolize peace and prosperity for the new reign.
It included a gold embroidered crown and the Royal Cipher EIIR.
The robe was edged with white ermine and gold lace.
"At other coronations the robe had been lined with ermine, but because there could have been a heatwave in June, the Queen had broken with tradition and chosen an oyster silk lining."

This pic is from a slim souvenir volume.
It looks as though three lengths, after cutting, were sewn together to achieve the width of the train.