This chapter focuses on the pioneering formulations of Frederick Douglass. It investigates earlier appearances of this important phrase: color line in discourses of abolition and liberty, ethnicity and activism. The chapter examines important nineteenth-century moments in which the term the color line was debated on both sides of the Atlantic. A key link between slavery, transatlantic abolition and Du Bois's invocation of the color line between 1897 and 1903 is an important 1881 essay by Frederick Douglass called The Color Line. The chapter argues that Du Bois's use of a term also used by Douglass reveals the complex ways in which black activists developed a set of transatlantic, transnational and international political alliances. The problem of interpreting Douglass's invocation of the color line brings to an important phase of the political activism to end slavery in the United States: the visits to Britain and Ireland in the 1840s to the 1860s of key African-American activists.