Find a convenient male adult of roughly average size. Measure the distance from his shoulder to his wrist. You should find that it is about 22–23 inches (56–58 cm). That’s one of the oldest ways to define an ell, once the usual measure in large parts of Europe for textiles such as woollen cloth. It was considered to be roughly equal to six hand-breadths — a hand was 4 inches, a unit still used for measuring the heights of horses, which would make an ell about 24 inches.
In Old English, ell meant the arm, so that the elbow is the arm bend. There was even an saying give him an inch and he will take an ell; when the ell was replaced by the yard, the saying changed, too.
In medieval England, the ell was fixed in size by various acts of Parliament to be 45 inches — twice the size of the older unit. Even this is open to some variation, as the same acts of Parliament defined an inch to be the breadth of the thumb, in particular that of the official, the alnager, whose job it was to measure and stamp each piece of cloth as conforming to the law. His name comes from Old French aulne, to measure by the ell, a word which has the same Indo-European root as Latin ulna, the forearm.
The alnager was an important part of medieval consumer protection, whose job was to protect buyers of woollen cloth (a key English export of the period) from fraud. Parliament passed several laws fixing his duties, most of them self-contradictory, by 1500 reaching a point at which it was hard to see how he could do much that was useful at all. The post was finally abolished in 1699, in the reign of William III.