In a traditional katana, only one side of the blade is sharpened, and that side is known as the cutting edge. The cutting edge is the side of the blade that is intended for making contact with the target during a strike or cut. It is typically the side that faces outward when the katana is worn or displayed.

The other side of the katana blade, known as the back edge or the spine, is not sharpened and remains thick and blunt in comparison to the cutting edge. The back edge provides structural support to the blade and adds rigidity.

It's important to exercise caution and handle a katana with care, regardless of which side is sharp. The cutting edge is extremely sharp and can cause serious injury if mishandled or used improperly. Respecting the weapon and adhering to proper sword handling techniques are crucial for maintaining safety and preventing accidents.