Negro and the Warsaw Ghetto", Du Bois wrote about his three trips to Poland, particularly his third in 1949, during which he viewed the remains of the Warsaw Ghetto. Du Bois wrote: The result of these three visits, and particularly of my view of the Warsaw ghetto, was not so much clearer understanding of the Jewish problem in the world as it was a real and complete understanding of the Negro problem. In the first place, the problem of slavery, emancipation and caste in the United States was no longer in my mind a separate and unique thing as I had so long conceived it. It was not even solely a matter of color and physical and racial characteristics, which was particularly a hard thing for me to learn, since for a lifetime the color line had been a real and efficient cause of misery. He goes on to write: "No, the race problem in which I was interested cut across lines of color and physique and belief and status and was a matter of cultural patterns, perverted teaching and human hate and prejudice, which reached all sorts of people and caused endless evil to all men." These quotations are of note because they reflect an expansion of Du Bois’ original definition of the color-line to include discrimination beyond that of color discrimination, Du Bois also pared down his definition to acknowledge that the "problem of the color-line" as he initially imagined it existed in the United States and did not manifest itself identically across the world. Though discrimination existed everywhere, Du Bois expanded his mindset to include discrimination beyond that of simply black versus white. Use in 20th-century literature and literary theory Both the quote and the phrase can be found in numerous texts of the 20th century, both academic and non-academic alike. Langston Hughes uses the phrase in his autobiography, writing: "In Cleveland, a liberal city, the color-line began to be drawn tighter and tighter. Theaters and restaurants in the downtown area began to refuse to accommodate colored people. Landlords doubled and tripled their rent at the approach of a dark tenant." Closer to the end of the twentieth century, Karla F.C. Holloway, a professor of English at Duke University, centered her keynote address to the National Conference of Researchers of English on this sentence, saying: "Perhaps while sitting in his den or maybe in the midst of