He was one of the first to be devoted to research on medieval music and his numerous publications focused on subjects such as the Gregorian chant, the neumatic and measured notation, medieval instruments, and the theory and polyphony he called "harmony". What distinguished Coussemaker from Fétis is the wide culture of the latter that enabled him to synthesise huge quantities of information in order to elaborate on abstract theories. De Coussemaker's approach is nonetheless more accurate, more scientific and more hypothetical.