Description: Sal discusses metric units of length or distance such as millimeter, centimeter, meter, and kilometer. Created by Sal Khan. The base unit for distance in the metric system is the meter. So, for example, in this drawing right over here by Leonardo da Vinci, if this is an average-sized male, a meter, it would be more than half way across his wingspan. And I just wrote a lowercase m here as really just a short way of writing meter.*Teacher Advisor is 100% free.