Actually I don't get it, and I'm hoping for some guidance.
I just used a DE, not for the first time in a long time, but for the first time in my life. My head is still atop my shoulders.
I noticed that on the blade (Merkur Super Platinum) the two edges were marked I and II. Is this so I can use up one side before using the other? Or perhaps to rotate each day? I never had much challenge concerning which side was which before. If I rotate my M3 180 degrees, I can tell after I shave that something was wrong.
All seriousness aside, I really am curious about the markings and about what, if anything, people do to keep track of which edge they are using.