Information regarding 'Dialects and sorobun' taken from "Tegami Koza Vol. 1" (volume 1, Lesson of writing a letter) edited by Tsutomu IGARASHI et al., issued by Heibonsha in 1935.
「方言と候文」に関しては五十嵐力他監修『手紙講座第1巻』平凡社、昭和10年より。