This document defines and provides background on several traditional units of measurement based on parts of the human body: - A cubit was the length from elbow to fingertip and varied from 18-22 inches depending on the person. - A digit can refer to a finger or toe. People use their digits to count to ten. - A fathom is about six feet, the length of an outstretched arm. It is used to measure water depth and areas of six square feet. - A hairbreadth was once a formal unit equal to 1/48 of an inch.

1. cubit – from Latin cubitum, “the elbow.” A cubit was a measurement
based on the forearm from elbow to fingertip. The exact length varied
according to whose arm was being used and could be from 18 to 22
inches.
digit – In Latin, digitus could mean either “finger” or “toe.” The same
is true of digit in modern English. People use the digits of their hands
to count to ten. And just think, we imagine we’re so modern and up-
to-date because we live in a Digital Age.
fathom – Old English faethm was “the length of the outstretched
arm,” about six feet. Water depth is measured in fathoms. Miners use
the term to describe an area equal to six square feet.
hairbreadth – According to the Online Etymology Dictionary,
hairbreadth is said to have once been a formal unit of measure equal
to one-forty-eighth of an inch.
hand – Originally, a “hand” was a measurement of three inches, but
now it is four inches. This measurement is stil used to reckon the
height of horses.
handful – This is an indeterminate quantity of some dry measure,
such as grain, that can be held in the cupped hand. A Greek word for
“handful” is the origin of the Greek coin called a drachma.
span – In Old English a span was “the distance between the thumb
and little finger of an extended hand,” roughly nine inches. Again, it
all depended on whose hand.
thumb was probably the basis of the measurement now called an inch.
We don’t measure things in “thumbs,” anymore, but we do talk
about thumbnail sketches and thumbnails in the sense of “small
images.”
The expression rule of thumb probably originated with carpenters
who used their thumbs to take rough measurements. The notion that
the expression originated with a law permitting a husband to beat his
wife with a stick “no larger in circumference than his thumb,” has no
historical basis.
Ancient Egyptians used elbow-to fingertip to measure distance. This is a
cubit.