Feet.

*the following is completely based on a scenario by my favorite brother-from-another-mother,  TechSmith's @troystein. Also, you may not understand the reason for this post and there's a reason for that. I promise I'm ok and I haven't lost my mind. But those of you who know, KNOW. Y'know? ;)

Good. Here we go: 

Suppose I have a piece of land and would like to build a house. I look at the layout of the property and, using MY feet (I'm 5'1.5" and wear a size 4 in kids shoes), I walk heel-to-toe to measure off the house I want. And based on my feet, I want a house that is 30 feet by 50 feet. And when I say "feet", I mean MY feet. This is a measurement of length I understand and with which I am familiar. 

I tell my contractor (who wears a size 11 mens shoe) the dimensions of the house I want. He goes to my property and, using HIS feet, walks heel-to-toe to measure out 30 feet by 50 feet. He then builds my house based on the length of his feet because that is a measurement of length he understands and is familiar with. 

He builds a house bigger than I anticipate and I can't afford to finish it. Why? Materials were purchased  based on HIS understanding of "feet". That's a different length than I understood for "feet".  

Obviously, we need a common length so that when we say "feet", we BOTH know how long that is and can clearly communicate about measurements.