WB's bisento, Murakumogiri, is one of the 12 supreme grade swords. Furthermore, a bisento is described as "a double-edged long sword with a thick truncated blade"
Its not one of the 12 Supreme grade swords, its one of the 12 Supreme grade blades.
Its more like a glaive, the style of using a bisento is completely different than that of swordsmanship.
Though even beyond that, WB doesn't seem like a swordsman the way Roger is, Roger who can perform pound ho like Zoro while WB is mostly a DF using brawler. Some of his strongest attacks have come without his blade.
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@Shiroyru
A bisentō was a pole weapon used in feudal Japan. The bisentō has various descriptions, "a double-edged long sword with a thick truncated blade", "a spear-like weapon with a blade at the end that resembles a scimitar", "a polearm resembling a glaive, with a long, heavy haft and a heavy, curved blade".
Its kinda tricky to define a bisento, but I would say its at most effective when used as a poking device like a spear hence the description spear like.
In a swordsmanship, at least speaking from logical stand point, its not a very useful weapon.