such as the range of subject matters that were deemed appropriate for merchant class boys and reinforced the notion that the primary purpose of akahon were to "entertain, rather than to inculcate or instruct". were preceded by ("mini reds") which had the same red cover and similar contents but measured only . Scholars speculate on the reasons why gradually replaced , with theories spanning from greater commercial viability as a book of bigger format to accommodating more visual-verbal content. An was typically just 10 pages long, though often, groups of up to 3 were collated and bound together with string. Slowly, would be overshadowed by emerging literary mediums, such as manga. However, cemented its place in Edo-Japan literature, as it was grouped together with other notable text types of kabuki and to be coined with the term that was seen to be "fashionable literature for a metropolitan audience." was the family of children's books to which belonged. There are several explanations for why they are named "grass books." One theory involves the etymology of the Chinese character for grass which has a secondary meaning of "crude", or "coarse", which describes many adults' opinion of at the time as a crass medium that adversely affected the juvenile audience. Another theory is that relates to the "grass style" of calligraphy with which the tales are inscribed. Other examples of include ('blue book'), ('yellow book') and ('black book'). Out of these three texts, is most notable, as many sources believe that modern manga was a direct descendant of the books with yellow covers. These texts were targeted at more mature audiences and their subject matter reflects this fact, as they covered the darker side of the human condition, such as mortality, infidelity, horror or hopelessness. Although the illustrations still occupied a central position, there was notably a greater didactic presence in these more mature texts and there would often be a philosophical moral to the story subscribing to the ideals of Confucianism or Buddhism, that led academics to conclude that an ulterior motive of was in the education of Japanese children. Soon after, publishers started to lengthen the structure of their stories, incorporating complex heroes and expansive plotlines such that the resultant work had the length of multiple standard leaflets, necessitating the coinage of the new term , meaning "bound volumes". A common source of literary inspiration of were the of the 17th