The primary difference between a tachi and a katana lies in their historical usage, design, and wearing style within the realm of Japanese swords.

Tachi:

• The tachi is an older style of Japanese sword that predates the katana.
• Tachis have a longer blade length compared to katanas, typically measuring around 70 to 80 centimeters (27.6 to 31.5 inches), although there can be variations.
• The curvature of the tachi blade is more pronounced compared to a katana, with a deeper curve that extends along the entire length of the blade.
• Tachis were historically used by samurai warriors, particularly during the Heian and Kamakura periods, and were commonly wielded on horseback.
• Tachis were worn with the cutting edge facing downward, known as "edge-down" orientation.
• The design and curvature of the tachi were optimized for effective cutting and slashing movements from horseback, as the curve allowed for longer reach and sweeping attacks.
• Tachis were often paired with a specialized suspension system called koshirae, which allowed for easy drawing and sheathing while mounted.

Katana:

• The katana is a traditional Japanese sword that emerged as a development of the tachi.
• Katanas have a shorter blade length compared to tachis, typically ranging from 60 to 70 centimeters (23.6 to 27.6 inches), although variations exist.
• The curvature of the katana blade is less pronounced compared to a tachi, with a gentler curve that starts closer to the tip of the blade.
• Katanas were the primary weapon of the samurai class during feudal Japan.
• Katanas were worn with the cutting edge facing upward, known as "edge-up" orientation, and were often paired with a smaller companion sword, such as a wakizashi, forming the daisho.
• The design and balance of the katana were optimized for versatile use, both on foot and horseback, with a focus on quick draw techniques and close-quarters combat.
• The katana's curved blade allowed for efficient cutting and slicing motions, while its balanced construction facilitated swift and agile movements.

In summary, the key differences between a tachi and a katana lie in their blade length, curvature, historical usage, and wearing style. Tachis have longer blades with more pronounced curves, were primarily used on horseback, and were worn with the cutting edge facing downward. Katanas have shorter blades with gentler curves, were used in various combat situations, and were worn with the cutting edge facing upward. The evolution from tachi to katana brought changes in design and usage, reflecting the shifting needs and martial traditions of samurai warriors over time.