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Writers' Guild of Great Britain was created in 1959.
Directors Guild of Great Britain was created in 1983.
Showmen's Guild of Great Britain was created in 1889.
Writer's is the singular possessive form of writer, e.g. I've got writer's cramp
Writers' is the plural possessive form of writers, e.g. The Writers' Guild of Great Britain
Little Theatre Guild of Great Britain was created on 1946-05-18.