Ku (Machi) and Machi-okuri: How Blade-ku and Ridge-ku Shapes Reveal the Era and Appraisal Significance of Japanese Swords

What is Ku (Machi)? — The Boundary Separating Blade and Tang

At the boundary between the blade and tang (nakago) of a Japanese sword, there is a step called "ku (machi)." This ku consists of two elements:

Blade-ku (ha-machi): The step on the edge side. The starting point where the blade edge begins.

Ridge-ku (mune-machi): The step on the ridge (back) side. The starting point where the ridge line begins.

The blade-ku and ridge-ku are positioned opposite each other, but they are not necessarily at the same height. The straight-line distance from blade-ku to ridge-ku (the width perpendicular to the blade) is sometimes called "ku-width (machi-haba)."

The ku is not merely a formal boundary but also serves as a "stop" that fixes the blade to the hilt (tsuka). In combination with the peg hole (mekugi-ana), it controls axial movement of the blade and is an important functional element in practical use.

Classification of Ku Shapes — The First Step in Reading Era

Ku shapes have characteristics that vary by era, region, and school, and serve as important criteria in appraisal. The main shape classifications are as follows.

Blade-ku Shapes

Right-angle ku (chokkaku-machi): The blade-ku is cut almost perpendicular to the blade axis. This is the standard form seen on swords of many eras.

Slanted ku (naname-machi): The blade-ku is set at an angle to the blade axis. Found on some swords from the old sword (ko-to) period.

Rounded ku (marumi-machi): The blade-ku has a rounded shape. Repeated polishing can also cause a right-angle ku to become rounded.

Ridge-drop ku (shinogi-ochi-machi): The ridge (shinogi) line drops near the ku. A unique form found on flat-forged swords.

Ridge-ku Shapes

Straight ridge-ku (sugu-mune-machi): The ridge-ku is set in a straight line. The most common form.

Inset ridge-ku (iri-mune-machi): The ridge-ku recesses slightly inward.

Protruding ridge-ku (hari-mune-machi): The ridge-ku protrudes slightly outward.

As a temporal trend, premium swords from the Kamakura period have clear and deep ku shapes. From the Muromachi period onward, mass-produced swords tend to have shallower ku shapes. Swords made with care in the Edo period again show clear ku shapes.

Machi-okuri — Evidence Carved by Polishing History

"Machi-okuri" is one of the important concepts in Japanese sword appraisal. This refers to the phenomenon where, as a result of polishing shortening the blade, the ku position relatively recedes toward the tang side.

More precisely: when polishing removes blade material in the direction of the blade-ku and ridge-ku, the ku itself "recedes" (appearing to move toward the tang). This is expressed as "the ku has been sent," and the degree is called "machi-okuri amount."

Methods for Determining Machi-okuri:

1. Confirm the direction and cutting condition of the blade-ku and ridge-ku surfaces. Large machi-okuri causes the ku surface to become dull.

2. Measure the distance from the tang's peg hole to the ku and compare with swords of the same era and type.

3. Confirm the positional relationship between the hilt and the ku. Swords with large machi-okuri may show unnatural fit with the hilt.

4. Observe shape changes in the transition from ku to tang.

Evaluation by Machi-okuri Amount:

• Little to no machi-okuri (less than 1mm): Minimal polishing history. Likely high blade integrity.

• Moderate machi-okuri (1–5mm): Appropriate maintenance polishing has been performed. Standard condition.

• Large machi-okuri (over 5mm): Evidence of repeated polishing. Possible shortening of blade length.

Large machi-okuri indicates blade shortening, which is distinguished from "suriage" (professional resurfacing). While suriage intentionally shortens the tang, machi-okuri is the result of blade material being removed through accumulated polishing.

Usage History Revealed Through Ku Observation

Information about the blade's usage history can also be obtained from the ku condition.

Ku of practical swords: Near the ku of swords actually drawn from their sheaths (saya), friction marks from the sheath's inner wall (just below the habaki ferrule) may be imprinted. When fine wear is visible, especially near the ridge-ku area corresponding to the lower edge of the habaki, it indicates frequent use.

Distribution of forging flaws: The ku area is where forging stress concentrates and where fine defects (forging flaws and fatigue) easily develop. Pay special attention to skin pattern disturbances from ku to blade-ku.

Rust condition: The preservation state can be determined by where rust begins on the blade. If rust originates near the ku, the area below the habaki may not have been properly dried and maintained.

Comprehensive Appraisal of Ku and Tang (Nakago)

It is important to understand the ku not in isolation but as an integrated part of the entire tang.

Continuity between blade-ku and file marks: When the tang's file mark pattern begins just below the blade-ku, the tang's original shape is likely preserved.

Relationship between peg hole position and ku: The distance from peg hole to ku (peg interval) shows consistent tendencies by era and school. If this distance is too long or short, suriage or later modification should be considered.

Consistency between signature (mei) and ku: The meaning differs depending on whether the signature is carved right next to the ku or at a distance. In swords with large machi-okuri, the ku may approach the signature too closely.

The ku, though a small feature, is a silent witness to a sword's history. By carefully observing its shape, condition, and position, the complete picture of polishing history, preservation condition, and usage from creation to the present emerges.