In Japan, since the eleventh or twelfth century, when teachers and students of the tanka (
, a short poem) recognized the importance of the teniwoha for expressing thoughts or feelings, its study became a vital part of kagaku (
, the study of verse-making). One of the earliest books on the matter, the “ Teniwoha Taigaisyô ” (
), was said to have been written by Huziwara-Sadaie (
, 1162–1241), a well-known poet of the thirteenth century. In the fifteenth and sixteenth centuries the study of the teniwoha made further headway, the reason being that the gap between the written and spoken language which had been developing since the eleventh century had widened so much that people found it difficult to understand the tanka of earlier times. There were at this time two groups of people who studied the teniwoha for the art of verse-making; one group was concerned with the tanka, and the other with the renga (
, a linked poem) which was then very popular. Although these studies of the teniwoha could be called the origin of the grammatical study of the Japanese language, they were very naive; all that the scholars did was to compile glossaries or reference book for verse-making.