In 1827 was the city still called Constantinople, or was that only the Greek name?
Short answerConstantinople remained widely used; the official nationalist shift toward Istanbul came in the twentieth century.
NotesPlace names can vary by language and political moment. Istanbul was already used, but Constantinople remained a normal name for the Ottoman capital in nineteenth-century European diplomacy. The Treaty of Constantinople therefore reflects contemporary usage rather than a purely Greek label. The formal nationalist push to standardize Istanbul came much later, during twentieth-century Turkification.