The present article deals with the methodology of considering the functional aspects of formal documentary styles in discussing the transformation that occurred in document forms in ancient and medieval Japan. The discussion focuses on the typically "medieval" form known as migyosho 御教書, a directive issued by a top ranking aristocratic (3^<rd> rank and above), written in the form of a letter and sent by his secretary. First the author takes up the ancient document form known as cho 牒, an initially ignored Chinese form of communication between officials of different government and religious organizations, arguing that migyosho functioned much like cho, except for the former's appearance as private correspondence. Next, the author distinguishes between the different usages of "private" cho between those directives directly issued (jikijo 直状) from the hands of aristocrats and those written by their secretaries (hosho 奉書), and points to the revision of the ritsuryo codes in AD 804 which recognized the legality of a document called keishi-cho 家司牒, which was signed by an aristocrat's secretary (keishi 家司) in his name and was based on the custom of aristocrats not bothering to sign day-to-day clerical documents. This revision, according the author, marked the origin of migyosho. Finally, the discussion turns to research the done to date, arguing that there has been a failure to distinguish between the use of migyosho as an historical term and as an analytical term, resulting in emphasis on the document's function, instead of its pre-defined hosho form. Therefore, since 1) the historical existence of "migyosho" cannot be traced back any earlier than the tenth century and 2) the last vestige of "private" cho appeared during the last half of the ninth century, the author hypothesizes that a transition between the two document forms probably took place between the late ninth and early tenth century. The paper concludes with the overall view that from as early as the Nara Period, the official documentation stipulated by the ritsuryo codes did not suit the day-to-day clerical tasks of administration, resulting in confusion about document forms. Then, during the late ninth century with a growing appreciation and understanding of "things Chinese, such document forms as kokusho 告書 and cho were reorganized in the process of the transition to medieval-style documentation. One of the social factors in the reorganization was the central role that would be played by documents in claiming and legitimizing power and authority.