The orihon structure was developed during the Heian period (794–1185), and was traditionally used for Buddhist sutras. It is thought that if an orihon book inscribed with Buddhist sutras is 'cascaded' from hand-to-hand, the draught created by the falling pages has healing properties if directed to the affected part of the sufferer's body. In Japanese Buddhist monasteries it is still sometimes possible to receive this treatment.
The orihon structure developed out of the scroll simply by folding the scroll into an accordion-fold and pasting it between covers. In this form it was both easier to access and to store and, although not yet truly a 'book-shaped' book this was the vital step in its development. Structure The basic orihon book consists simply of an accordion-folded sheet of paper bound within soft or hard covers. The paper can be a single long strip, or several smaller strips pasted together. Traditionally, smaller sheets would be pasted into a long strip with little regard paid to where the joins fell; as the style developed, however, the joins were usually planned to fall at the page folds.
If the flaps all fall at the same edge, the book will have greater thickness one side than the other; for this reason if there are to be a great number of joins they are sometimes placed at the back- and fore-edges alternately, even though the back-edge joins are more visible than the fore-edge joins. If an even number of pages is used, the book will have a definite 'front' and 'back'; if an odd number of pages is used, the concepts of 'frontness' and 'backness' may be more equivocal – either doubling the capacity of the book, or enabling a double book not dissimilar to the Western dos-à-dos structure.