Typological studies have focused thus far on the universals and variations of word orders. Classical language typology lays the foundation for contemporary language typology. This chapter explores the theoretical principles and explanations for disharmonic phenomena in word order typology mainly based on the seminal works of Greenberg (1963, 1966), Lehmann (1973,1978), Vennemann (1973, 1974a, 1974b, 1976), Hawkins (1983), and Dryer (1992, 1995).Furthermore, we seek possible explanations for the basic patterns of word order from the semantic,syntactic, linear and diachronic perspectives, respectively, and delineate the word order parameters, the research scope and data adopted in this book.