The cubit is an ancient unit of length based on the distance from the elbow to the middle finger. It's primarily associated with the Sumerians, Egyptians and Israelites. "Cubits" is found in the Bible re: Noah's Ark, Ark of the Covenant, Tabernacle, Solomon's Temple. The common cubit was divided into 6 palms × 4 fingers = 24 digits. Royal cubits added a palm for 7 palms × 4 fingers = 28 digits. These lengths typically ranged between 444 and 529.2 mm (17.48 and 20.83 in), with an ancient Roman cubit being as long as 120 cm (47 in).
Cubits of various lengths were employed in many parts of the world in antiquity, during the Middle Ages and as recently as early modern times. The term is still used in hedgelaying, the length of the forearm being frequently used to determine the interval between stakes placed within the hedge.
The ancient Egyptian royal cubit (meh niswt) is the earliest attested standard measure. Cubit rods were used for the measurement of length. A number of these rods have survived: two are known from the tomb of Maya, the treasurer of the 18th dynasty pharaoh Tutankhamun, in Saqqara; another was found in the tomb of Kha (TT8) in Thebes. Fourteen such rods, including one double cubit rod, were described and compared by Lepsius in 1865. These cubit rods range from 523.5 to 529.2 mm (20.61 to 20.83 in) in length and are divided into seven palms; each palm is divided into four fingers, and the fingers are further subdivided.
Hieroglyph of the royal cubit, meh niswt
Early evidence for the use of this royal cubit comes from the Early Dynastic Period: on the Palermo Stone, the flood level of the Nile river during the reign of the Pharaoh Djer is given as measuring 6 cubits and 1 palm. Use of the royal cubit is also known from Old Kingdom architecture, from at least as early as the construction of the Step Pyramid of Djoser designed by Imhotep in around 2700 BC.
Ancient Mesopotamian units of measurement originated in the loosely organized city-states of Early Dynastic Sumer. Each city, kingdom and trade guild had its own standards until the formation of the Akkadian Empire when Sargon of Akkad issued a common standard. This standard was improved by Naram-Sin, but fell into disuse after the Akkadian Empire dissolved. The standard of Naram-Sin was readopted in the Ur III period by the Nan?e Hymn which reduced a plethora of multiple standards to a few agreed upon common groupings. Successors to Sumerian civilization including the Babylonians, Assyrians, and Persians continued to use these groupings.
The Classical Mesopotamian system formed the basis for Elamite, Hebrew, Urartian, Hurrian, Hittite, Ugaritic, Phoenician, Babylonian, Assyrian, Persian, Arabic, and Islamic metrologies.[full ] The Classical Mesopotamian System also has a proportional relationship, by virtue of standardized commerce, to Bronze Age Harappan and Egyptian metrologies.
In 1916, during the last years of the Ottoman Empire and in the middle of World War I, the German assyriologist Eckhard Unger found a copper-alloy bar while excavating at Nippur. The bar dates from c. 2650 BC and Unger claimed it was used as a measurement standard. This irregularly formed and irregularly marked graduated rule supposedly defined the Sumerian cubit as about 518.6 mm (20.42 in).
The standard of the cubit (Hebrew: ) in different countries and in different ages has varied. This realization has led the rabbis of the 2nd century CE to clarify the length of their cubit, saying that the measure of the cubit of which they have spoken "applies to the cubit of middle-size". In this case, the requirement is to make-use of a standard 6 handbreadths to each cubit, and which handbreadth was not to be confused with an outstretched palm, but rather one that was clinched and which handbreadth has the standard width of 4 fingerbreadths (each fingerbreadth being equivalent to the width of a thumb, about 2.25 cm). This puts the handbreadth at roughly 9 centimetres (3.5 in), and 6 handbreadths (1 cubit) at 54 centimetres (21 in). Epiphanius of Salamis, in his treatise On Weights and Measures, describes how it was customary, in his day, to take the measurement of the biblical cubit: "The cubit is a measure, but it is taken from the measure of the forearm. For the part from the elbow to the wrist and the palm of the hand is called the cubit, the middle finger of the cubit measure being also extended at the same time and there being added below (it) the span, that is, of the hand, taken all together."
Rabbi Avraham Chaim Naeh put the linear measurement of a cubit at 48 centimetres (19 in). Avrohom Yeshaya Karelitz (the "Chazon Ish"), dissenting, put the length of a cubit at 57.6 centimetres (22.7 in).
Rabbi and philosopher Maimonides, following the Talmud, makes a distinction between the cubit of 6 handbreadths used in ordinary measurements, and the cubit of 5 handbreadths used in measuring the Golden Altar, the base of the altar of burnt offerings, its circuit and the horns of the altar.
In ancient Rome, according to Vitruvius, a cubit was equal to 1+1⁄2 Roman feet or 6 palm widths (approximately 444 mm or 17.5 in). A 120-centimeter cubit (approximately four feet long), called the Roman ulna, was common in the Roman empire, which cubit was measured from the fingers of the outstretched arm opposite the man's hip.; also, with
In the Islamic world, the cubit (dhir) had a similar origin, being originally defined as the arm from the elbow to the tip of the middle finger. Several different cubit lengths were current in the medieval Islamic world for the unit of length, ranging from 48.25 centimetres (19.00 in) to 145.6 centimetres (57.3 in), and in turn the dhir was commonly subdivided into six handsbreadths (qab?a), and each handsbreadth into four fingerbreadths (a?ba?). The most commonly used definitions were:
A variety of more local or specific cubit measures were developed over time: the "small" Hashemite cubit of 60.05 centimetres (23.64 in), also known as the cubit of Bilal (al-dhir al-Bil?liyya, named after the 8th-century Basran q Bilal ibn Abi Burda); the Egyptian carpenter's cubit (al-dhir bi'l-najj?ri) or architect's cubit (al-dhir al-mi?m?riyya) of c. 77.5 centimetres (30.5 in), reduced and standardized to 75 centimetres (30 in) in the 19th century; the house cubit (al-dhir al-d?r) of 50.3 centimetres (19.8 in), introduced by the Abbasid-era q Ibn Abi Layla; the cubit of Umar (al-dhir al-?Umariyya) of 72.8 centimetres (28.7 in) and its double, the scale cubit (al-dhir al-m?z?niyya) established by al-Ma'mun and used mainly for measuring canals.
In medieval and early modern Persia, the cubit (usually known as gaz) was either the legal cubit of 49.8 centimetres (19.6 in), or the Isfahan cubit of 79.8 centimetres (31.4 in). A royal cubit (gaz-i sh?h?) appeared in the 17th century with 95 centimetres (37 in), while a "shortened" cubit (gaz-i mukassar) of 6.8 centimetres (2.7 in) (likely derived from the widely used cloth cubit of Aleppo) was used for cloth. The measure survived into the 20th century, with 1 gaz equal to 104 centimetres (41 in). Mughal India also had its own royal cubit (dhir-i p?dish?h?) of 81.3 centimetres (32.0 in).
The 18th century physician and antiquarian William Stukeley proposed that a unit he called the "Druid's cubit" had been used by the builders of megalithic monuments such as Stonehenge and Avebury. Stukeley's cubit was 20.8 English inches (530 mm) in length, a measure whose multiples he claimed to detect in the dimensions of ancient structures. 
Other measurements based on the length of the forearm include some lengths of ell, the Chinese chi, the Japanese shaku, the Indian hasta, the Thai sok, the Malay hasta, the Tamil "(Mulzham)", the Telugu "(Moora)" (), and the Khmer hat.
A cubit arm in heraldry may be dexter or sinister. It may be vested (with a sleeve) and may be shown in various positions, most commonly erect, but also fesswise (horizontal), bendwise (diagonal) and is often shown grasping objects. It is most often used erect as a crest, for example by the families of Poyntz of Iron Acton, Rolle of Stevenstone and Turton.
On the roadside the finish is clean and neat, a living fence of intertwined branches between stakes placed an old cubit (the length of a man's forearm or roughly 18 inches) apart.
Solinus, cap. 45, uses ulna for cubitus, where Pliny speaks of a crocodile of 22 cubits long. Solinus expresses it by so many ulnae, and Julius Pollux uses both words for the same... they call a cubitus an ulna.
... Roman ulna of four feet...