Fair point. Metal bodied planes were probably a bad example to use as they are a pretty recent innovation in the west.
I always thought katanas were made from a single steel blank. The spine (less hard) and edge (harder) are hardened to different levels allowing it to take a keen, durable edge while also allowing the spine to flex. A cool Japanese innovation.
Until recently, I didn’t realize that it was this differential heat treatment that gave the katana its signature curve.
Do I have something wrong here?