The span of storeyed tower decrease in proportion to upper storeies. Generaly the rate of decrese is high in the ancient tower, and it is low since the mediaeval tower. But the rate of decrease in the modern tower does not nearly deffer from the mediaeval tower. Because, since in the mediaeval age the span is set on the basis of Shiwari, and so the decrease is beyond its control by the limitation of the rafter's number. It is later than the architecture of single storey that the rule of Shiwari is completed about the tower. It is difficult that unify several storeyed spans by same Shiwari taking the decrease of each storey into account. In case of five storeyed-tower, at the bigining the rule of Shiwari is used only first storey for example Kaijyusenji tower, next Shiwari of first storey used only to decide full span (span consits of a central and tow side) in over second storey for example Myooin tower, lastly all storey's span is unifyed by same Shiwari for example Itsukushima tower. But several tower use transitional way (each sotrey uses individual Shiwari) until the middle of 15 C. In case of three storeyed-tower, as number of storey is more few than five storeged tower, Shiwari is completed easily.
