BBC News
By David Wilson
image copyrightImage Courtesy of the National Library of Ireland
When the Government of Ireland Act was granted Royal Assent in December 1920, it came at the end of a troubled year in Londonderry.
Six months earlier in the city, a single week of violence had claimed 20 lives.
It was the "first upsurge of sectarianism", according to Ulster University historian Dr Adrian Grant.
That week in June, he said, was "a hugely significant but largely forgotten time".
image copyrightThe Print Collector/Print Collector/Getty Images
image captionMuch of the violence occurred in the Bishop Street area
The city's electorate had returned a Sinn Féin MP in the 1918 Westminster elections.