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In page The Book of Five Rings:

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The Book of Earth ends with nine basic principles – the "ground" upon which the samurai must rely. These are "practical" or "worldly," each intended to help develop an understanding of strategy (while the other books focus on tactics and movement). These rules are for his students, and are complemented by the 21 "spiritual principles" for all to follow, which are found in the Dokkōdō (Musashi's final work). The principles are:[citation needed]