The groove down a sword blade
A long channel runs down the flat of many swords and knives.
Category: Desk & tools
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Feature. It makes the blade lighter and stiffer.
This groove is called a fuller. Like the flange on an I-beam, it lightens a blade without losing much stiffness.
The myth: It is often called a "blood groove" and said to let blood run out. There is no good evidence for that purpose.
Source: Fuller (weapon), Wikipedia.