Kinsen (金線) is one of the features that form the blade pattern of a Japanese sword. It refers to a black, shining line that runs along the grain of the blade, within the ""Nie"" (沸) or the ""Nioi"" (匂). The ""Nie"" refers to the visible particles of hardened steel that appear along the cutting edge of the blade, while the ""Nioi"" refers to the whitish misty appearance also along the cutting edge, which is a more subtle hardening compared to ""Nie"". Both ""Nie"" and ""Nioi"" are results of the differential hardening process in sword making.

The ""Kinsen"" is of the same quality as the ""Inazuma"" (稲妻) that appears in the blade, and the ""Chikei"" (地景) that appears in the ""Jigane"" (地鉄), which is the body of the blade. The ""Inazuma"" is a pattern that appears as a thin, black, shining line that seems to cut through the ""Nie"" or the ""Nioi"", resembling lightning streaking through clouds. The ""Chikei"" is a pattern that appears as dark lines or spots on the surface of the blade, resembling landscapes.

A ""Kinsen"" that has a dull shine is referred to as a ""Ginsen"". The ""Imonotsuru"" (芋の蔓), which is often seen in Satsuma swords, is a large feature of this ""Kinsen"", and it appears as a continuous line. It is said that the ""Kinsen"" is a feature that appears as a result of the swordsmith's skill.

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