An Ell is a unit of measure used by early Scandinavians and was concidered an ancient measure in the Viking Age (800-1200 CE). Ell translates as 'arm' and so thus was the length of your arm. This is in no way related to the Latin Ulna, elbogi (elbow) simply means the bow or bend (bogi) in your arm (ell) and is derived from Old Norse, not Latin.
So an arm's length could be measured in a few different ways. The standard, in the Viking age, was from the tip of your finger to your shoulder or armpit. Just for references, a personal ell for me is 30 and 1/4 inches. As I am a bit taller then the average man in the viking age, my ell is longer then average.
This difference between the measure of men was known and so led to the custom of "giving an inch". This practice gave leeway of an inch (the width of a thumb in Viking Age) either way for purposes of trade. This lead to the creation of the yard as the Scandinavians of that time were noticeably bigger then the British and continental Europeans that they were trading with. Some report the yard as a measure the length of a mans belt. The yard measures the fingertip to the middle of the chest, so as to match more closely with the Norse ell. This is sometimes also called a Scottish ell (1 yard).
At this time the English began to measure from the fingertip to the opposite shoulder, Thus increasing the English ell to be longer even then the yard. Not to be outdone the French began to use a measure from fingertip to fingertip of outstretched arms, minus a cubit. So a French ell was from one fingertip to the opposite elbow. These were used primarily for the cloth trade.
All measures were usually standardized by the local ruler or chieftain, later by kings and after the conversion of most of Europe to Christianity, the measures were based, in 1354, on those of Pope Innocent the VI of the Avignon Papacy.
The term Give an inch and he takes an ell is an old saying. The earliest recorded usage of this comes from the 14th century. After the ell fell from common use, the phrase was amended to 'give an inch and he takes a mile' this changes the context a good deal, but it carries the same general meaning.
The following website offers a great graphic right at the top that illustrates some of these.
http://eia.egreen.wed...s_of_measurements.htm