Palm (unit)
The palm is an obsolete anthropic unit of length, originally based on the width of the human palm and then variously standardized. The same name is also used for a second, rather larger unit based on the length of the human hand. The width of the palm was a traditional unit in Ancient Egypt, Israel, Greece, and Rome and in medieval England, where it was also known as the hand, handbreadth, or handsbreadth. The length of the hand—originally the Roman "greater palm"—formed the palm of medieval Italy and France. In Spanish customary units  or  was the palm, while  was the span, the distance between an outstretched thumb and little finger. In Portuguese  or  was the span. History

Ancient Egypt

The Ancient Egyptian palm () has been reconstructed as about . The unit is attested as early as the reign of Djer, third pharaoh of the First Dynasty, and appears on many surviving cubit-rods. The palm was subdivided into four digits () of about . Three palms made up the span () or lesser span () of about . Four palms made up the foot () of about . Five made up the  of about . Six made up the "Greek cubit" () of about . Seven made up the "royal cubit" () of about . Eight made up the pole () of about . Ancient Israel

The palm was not a major unit in ancient Mesopotamia but appeared in ancient Israel as the , , or  (, . "a spread"). Scholars were long uncertain as to whether this was reckoned using the Egyptian or Babylonian cubit, but now believe it to have approximated the Egyptian "Greek cubit", giving a value for the palm of about . As in Egypt, the palm was divided into four digits ( or ) of about  and three palms made up a span () of about . Six made up the Hebrew cubit ( or ) of about , although the cubits mentioned in Ezekiel follow the royal cubit in consisting of seven palms comprising about . Ancient Greece 

The Ancient Greek palm (, palaistḗ, , dō̂ron, or , daktylodókhmē) made up ¼ of the Greek foot (poûs), which varied by region between . This gives values for the palm between , with the Attic palm around . These various palms were divided into four digits (dáktylos) or two "middle phalanges" (kóndylos). Two palms made a half-foot (hēmipódion or dikhás); three, a span (spithamḗ); four, a foot (poûs); five, a short cubit (pygōn); and six, a cubit (pē̂khys). The Greeks also had a less common "greater palm" of five digits. Ancient Rome 

The Roman palm () or lesser palm () made up ¼ of the Roman foot (), which varied in practice between  but is thought to have been officially . This would have given the palm a notional value of  within a range of a few millimeters. The palm was divided into four digits () of about  or three inches () of about . Three made a span ( or "greater palm") of about ; four, a Roman foot; five, a hand-and-a-foot () of about ; six, a cubit () of about . Continental Europe

The palms of medieval () and early modern Europe—the Italian, Spanish, and Portuguese  and French —were based upon the Roman "greater palm", reckoned as a hand's span or length. In Italy, the palm () varied regionally. The Genovese palm was about ; in the Papal States, the Roman palm about  according to Hutton but divided into the Roman "architect's palm" () of about  and "merchant's palm" () of about  according to Greaves; and the Neapolitan palm reported as  by Riccioli but  by Hutton's other sources. On Sicily and Malta, it was . In France, the palm ( or ) was about  in Pernes-les-Fontaines, Vaucluse, and about  in Languedoc. Palaiseau gave metric equivalents for the palme or palmo in 1816, and Rose provided English equivalents in 1900:

From 19th C. Italian sources    emerges that :

- the ancient Venetian palm, five of which made a passo (pace), was equivalent to 0.3774 metres. - the Neapolitan palm = 0.26333670 metres (from 1480 to 1840)  

- the Neapolitan palm = 0.26455026455 metres (according to the law of 6 April 1840)

which differs from previously cited palm measure equivalents in metres above. England

The English palm, handbreadth, or handsbreadth is three inches (7.62cm) or, equivalently, four digits. The measurement was, however, not always well distinguished from the hand or handful, which became equal to four inches by a 1541 statute of Henry VIII. The palm was excluded from the British Weights and Measures Act of 1824 that established the imperial system and is not a standard US customary unit. Elsewhere
The Moroccan palm is given by Hutton as about . Notes

References 

Units of length
Human-based units of measurement
Obsolete units of measurement