The time I learned that there are two different systems for the clippers used to cut hair

This happened when I was still in the Air Force, so late 80’s.

I had been getting my haircut by someone off base and liked what he did. He told me he used a #2 clipper.

Sometime later I needed to get a haircut quick so I went to the barber on base and told her I use a #2 clipper.
She started with the back and when she came around the side I gasped and said “That is SHORT !! “

Her response was “You said #2!! You said #2!!”. Yes I did. 

And that is the day I learned to use length – 1/2” – vs trusting the numbering of the clippers when specifying how short to cut my hair.