My understanding ( until I'm corrected by a Dr Stephen K Hayes type ) is that a traditional Ninja Sword was for some reason straight in its spine - no bend.
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Whilst the Samurai sword had a definite bend in it ( Ooooh-errrr Missus... )
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This maybe true or it maybe Hollywood BS. I know there were various stated reasons why a straight sword was more use to a super secret spy type but they all sound a little BS now. When I was a child however I lapped it up. Because you're allowed to do so as a Child.
If you look at the weapons the Shinobi / Ninja / Black PJ night freaks had they were very varied and mostly seemed to have a previous agricultural type heritage