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The Ri (里) is a traditional Japanese unit of long distance, belonging to the Shakkanhō (尺貫法) system. It is defined as being equal to 36 chō (町).

Based on the standardized chō (which is 60 ken, with 1 ken = 6 shaku, and 1 shaku = 10/33 meters), the modern standardized Ri is equivalent to:

• 36 chō = 36 * (60 ken) = 36 * 60 * (6 shaku) = 12,960 shaku
• 12,960 * (10/33) meters = 129,600 / 33 meters = 43,200 / 11 meters
• Exactly 3927.2727... meters
• Approximately 3.927 kilometers (km)
• Approximately 2.440 miles (mi)

Historically, the Ri represented a significant travel distance, often conceptualized as roughly the distance a person could walk in one hour.

• Origin: The Japanese Ri unit, like many elements of the Shakkanhō measuring system, originates from ancient China, where the equivalent unit is the Li (里). This traditional unit of distance was adopted into Japan centuries ago.
• Variable Length: For much of Japanese history, the precise length of the Ri was not consistently fixed. Its value varied significantly depending on the region, the era, and even the specific context (e.g., measuring distance on flat land versus mountainous terrain). Different feudal domains (han) might use slightly different standards. For instance, during the Edo period (1603–1868), while a standard of 36 chō per Ri was common, local variations persisted, making historical distance conversions complex.
• Standardization (Meiji Era): Following the Meiji Restoration, Japan undertook efforts to modernize and standardize its systems, including weights and measures. The Weights and Measures Act of 1891 officially defined traditional Japanese units in relation to metric standards. The base unit shaku was set at 10/33 meters, which in turn fixed the values of the ken, chō, and consequently the Ri. The Ri was officially standardized as 36 chō, leading to the modern, precise value of 43,200/11 meters (approximately 3.93 km).
• Metrication: Japan officially adopted the metric system in 1951, with full implementation for official and commercial transactions mandated in 1959. This shift means the Ri became largely obsolete for practical, everyday measurements, replaced by the kilometer.

While the kilometer (km) is the standard unit for measuring long distances in modern Japan, the traditional Ri unit retains significance in specific contexts:

• Historical Context: Understanding the Ri is essential for interpreting historical travel accounts, old maps (like those showing the Tōkaidō road), and administrative divisions from feudal Japan. Landmarks called ichirizuka (一里塚 - "one-ri mound") were placed along major highways during the Edo period to mark distances in Ri.
• Literature and Culture: The Ri appears frequently in classical Japanese literature, poetry (like haiku), and folklore, often used to denote long journeys or significant, sometimes metaphorical, distances.
• Place Names: Many place names across Japan incorporate "Ri," often stemming from historical distance markers or locations whose names indicated their distance in Ri from a provincial capital or castle.
• Idioms and Proverbs: The unit features in well-known sayings, such as 「千里の道も一歩から」 (Sen ri no michi mo ippo kara - "A journey of a thousand ri begins with a single step"), emphasizing that even great undertakings start with small actions.
• Figurative Language: Speakers might use Ri figuratively to imply a very long way, a great effort, or a significant difference.

It is crucial to remember that the Ri is not used for contemporary official measurements, modern road signs (which universally use kilometers), or everyday distance calculations in Japan today.