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The Ken (間) is a traditional Japanese unit of length, primarily used in architecture and related fields. It is fundamentally defined as being equal to 6 Shaku (尺). Based on the modern standardized kanejaku Shaku (10/33 meters), the Ken is equivalent to:

• 6 * (10/33) = 60/33 = 20/11 meters
• Approximately 1.818 meters
• Approximately 5.965 feet

Historically, the Ken represented the interval between pillars in traditional Japanese buildings and served as a fundamental module for architectural design, influencing room sizes and the dimensions of elements like tatami mats.

The concept of the Ken, like the Shaku, originated from Chinese architectural practices (where the equivalent unit is Jian 間). It was adopted into Japan along with other elements of Chinese culture and measurement systems.

• Early Use: The Ken was used early on as a standard interval in temple and palace construction. Its length, however, was not strictly fixed and could vary based on the specific building style, region, or period. It was often tied directly to the prevailing length of the Shaku.
• Standardization Attempts: Over time, particularly during the Edo period (1603–1868), attempts were made to standardize measurements. The Ken became strongly associated with architectural modules. Two prominent regional standards emerged:
  • Kyōma (京間): The Kyoto module, typically based on a slightly larger Ken (often around 6.5 Shaku or based on tatami dimensions), leading to larger room sizes.
  • Inakama (田舎間) / Edoma (江戸間): The rural or Edo (Tokyo) module, typically adhering more closely to the 6 Shaku standard or sometimes slightly smaller (e.g., 6.3 Shaku), resulting in slightly smaller rooms. This became more common.
• Meiji Restoration (1868 onwards): With the standardization of the Shaku at 10/33 meters in 1891, the Ken was implicitly standardized as 6 Shaku = 20/11 meters (approx. 1.818 m). This is the value generally understood as the modern Ken.
• Metrication: Although Japan officially adopted the metric system, the Ken remains a crucial unit in traditional architecture and related fields.

The Ken's primary importance lies in its role as an architectural module:

• Traditional Japanese Architecture: It defines the standard bay (pillar-to-pillar distance) in the ken module system (kenshin system 間真). This dictates the layout and proportions of temples, shrines, traditional houses (minka), teahouses, and castles. Room sizes are often described in terms of tatami mats, whose standard dimensions (historically 1 Ken by 0.5 Ken) are derived from the Ken.
• Construction and Carpentry: Used for planning layouts, cutting timber, and sizing building components like fusuma (sliding doors) and shōji (screens).
• Real Estate and Land: Room sizes in Japan are often measured by the number of tatami mats they can hold (e.g., a "6-mat room" - 六畳 rokujō), which indirectly relates to the Ken. Historically, it was also used for land parcel dimensions.
• Martial Arts: The concept of distance or interval (maai 間合い) between opponents is crucial and sometimes measured or conceptualized in terms of Ken. The length of certain weapons, like the bō (staff), is often around 1 Ken (6 Shaku).
• Traditional Garden Design: Used for establishing proportions and spacing elements within Japanese gardens.
• Cultural and Historical Context: Understanding the Ken is essential for interpreting historical architectural plans, documents, and cultural descriptions of space.