Wichian Sittiprapaporn. (1997) A Thai dialect geography of Udonthani province : a tonal study. Mahidol University,:ม.ป.ท.Wichian Sittiprapaporn. 1997. A Thai dialect geography of Udonthani province : a tonal study. ม.ป.ท.:Mahidol University,;Wichian Sittiprapaporn. A Thai dialect geography of Udonthani province : a tonal study. ม.ป.ท.:Mahidol University, 1997. Print.