My dad taught me to use my hand’s span as a useful approximation of length. His span, when he stretched his thumb and his pinky as far as they could reach, was 9 inches. Before he moved a dresser, he’d measure the dresser in handspans and then measure the wall space to see how well it would fit. If he wanted an accurate measurement, he’d use a tape measure. But for “ballpark” calculations, he used his hand.
When I was growing up, I often measured my handspan. As I grew, it grew. I went from skip-counting by 6 to 7 to 8 inches. By the time I was 14, my handspan was 9 inches, just like my dad’s was. For a woman, I have huge hands with very long fingers. Excellent measuring devices. I often skip count by 9’s as I measure.
But saying, “This table is 5 handspans long” does not provide useful information unless everyone agrees on the definition of one handspan. Since human hands vary tremendously, how could there be a single accurate measurement called a span? Or a finger? A palm? A foot? Someone had to define each measure, and make sure that the relationship between them was constant.
I just discovered that the handspan I inherited from my dad is 27 barleycorns in length: a true span in the English Imperial System.
Knowing that, I could calculate the length of a fathom, a furlong, or even a league. But I’d probably make an error in calculation.
Look at those simple metric measures along the righthand side of the chart. Each level increases by a power of ten.
My handspan is 23 centimeters long. I could learn to count by 23’s.