The megalithic Yard is a unit of measurement of 2.72 imperial feet, or 82.9 cm, accurate to near (~), which would have been used by the “megalithic peoples” of the British Isles and Brittany.

The idea of a standard measure for the construction of megalithic sites goes back at least to, with William Stukeley who in 1740 during his study of Stonehenge suggests the use of a cubit of about 53 cm for the construction of the monument. It is then repeated by other authors such as Rene Kerviler, which in 1905 inclines for a measure of about 26 m divided into 48 cubits of about 53-54 cm for the megalithic monuments of Armorica. However, these are based on the study of a site or a limited number of sites. In 1955 Alexander Thom, a Scottish engineer, proposed a unit of measurement, the megalithic fathom, based on the systematic study of many sites in Great Britain, from careful field surveys with theodolite and Surveyor’s chain and specific plans that he establishes. In this megalithic breeze of 5.44 feet, he prefers in a 1962 article a main unit two times smaller of 2.72 feet which he calls the megalithic yard, and which he judges used with a precision close to a thousandth, to near (~). For Thom the determination of his standard unit, based essentially on a statistical analysis of the diameters of hoarded stone circles (cromlech), was established in 1955, and he would then content himself with “verifying” the use of this unit for his later records. , which involves much simpler statistical tools. This proposition of Thom is closely related to that of a geometry based on the Pythagorean triangles for the construction of these circles. The high degree of precision that Thom supports is not compatible with a diffusion by successive copies of standard rules, but supposes, as he writes himself, a centralized organization, capable of producing and disseminating these standard rules through The British Isles and Continental Brittany A. Thom is not the first to hypothesize a “megalithic” unit of measure, but Thom’s research methods have revived interest in astronomical interpretations of prehistoric monuments . Other systematic surveys were carried out by Scottish Miln, Schuchardt of Germany and Mondrijan of Austria. Thom’s conclusions were invalidated after new statistical analyzes and the identification of certain biases in the choice of his data. However, the widespread use in the megalithic era of a system of (less precise) measurements based on the human body remains possible.

This hypothesis is difficult to envisage for the scientific community, which does not recognize the possibility of a unit of measurement of such widespread and specific precision at that time. However, given the very large number of measures, it is possible to include a margin of error sufficient to decide the question or not. This was done by the statistician D. G. Kendall of the University of Cambridge in 1974, concluding that there are 99 chances out of 100 that the megalithic yard has existed. In 1983, Gérardin challenged Kendall’s model. It should be noted that neither Kendall nor Gérardin studied the records of Professor A Thom and his son on the major megalithic site of Carnac, where he noted that the gap between the lines of Menec West and Kermario is a whole number of 8,10, 12 or 14 megalithic yards.

Report on the Carnac Alignments project, Jean Pierre Mohen.