Comment by mdellabitta

I was taught basic counting, addition, and subtraction using a finger-abacus system called Chisanbop: https://en.wikipedia.org/wiki/Chisanbop

Now I'm plagued by off-by-one errors in my mental math, particularly when it comes to intervals. If you ask me how many days there are between now and Christmas, I will be uncertain about the answer unless I individually count them all off on my fingers.