Thread: What is the line running down the length of the blade?

I tried a search in the forum but didn't see this addressed.
When you look at the blade in the light and tilt it you can see a "line" running along the length of the blade about 3/16" from the edge.
A friend told me he had heard this had something to do with the quenching process when the blade was manufactured or something and I was curious if anyone here could educate us on what that line is exactly?
If this is the wrong forum for this topic I apologize. I wasn't sure whether to post it here or the "Razors" forum.

Thanks