Short answers, pulled from the story.
The nadir of American race relations is a historical period identified by scholar Rayford Logan as the worst time for race relations in the United States following the Civil War. It was characterized by the legal solidification of Jim Crow laws, widespread lynchings, racial massacres, and the systematic disenfranchisement of Black Americans through legislation passed between 1890 and 1908.
Rayford Logan coined the term in his 1954 book The Negro in American Life and Thought: The Nadir, 1877-1901. He defined the period as the time when "the Negro's status in American society" reached its lowest point after the Civil War.
Logan placed the nadir from 1877 to 1901. Other historians disagreed: John Hope Franklin and Henry Arthur Callis argued for dates as late as 1923. James Loewen proposed that the true nadir began around 1890, when Northern Republicans stopped supporting Southern Black people's rights, and lasted until the American entry into World War II in 1941.
The NAACP calculated that between 1889 and 1922, almost 3,500 people were victims of lynching, nearly all of them Black men. Ida Bell Wells-Barnett conducted one of the first systematic studies of lynching during this period, documenting that victims were killed for offenses as minor as being perceived as disrespectful to white people.
The 1921 Tulsa race massacre occurred in Tulsa, Oklahoma, when white mobs invaded and burned the Greenwood district. 1,256 homes were destroyed and 39 people were confirmed killed, with 26 of the victims being Black. It is one of the deadliest episodes of racial violence during the nadir period, and recent investigations suggest the Black death toll was considerably higher than official counts.
In 1896, the Supreme Court ruled in Plessy v. Ferguson that "separate but equal" facilities for Black people were constitutional; the Court at that time was made up almost entirely of Northerners. That ruling stood for 58 years until Brown v. Board of Education overturned it in 1954, the same year Logan published the book that gave the period its name.