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As with other weights and measures, our units of length underwent numerous changes over the centuries. Originally, the mile was the Roman "mille passus" or a thousand double steps of five feet each, making five thousand feet to the mile. There was also a yard of three feet. The uncia or inch as it became known later was the twelfth part of a foot. In the early Gallic system Ireland inherited, there were three of these to a yard and two yards to a fathom (later called a staff), then ten of those to a chain, ten chains to a furlong, and ten furlongs to a mile, or 6000 feet altogether. The thousandth part of a square mile, thirtysix thousand square feet, was an acre. While the relationships were fixed, the absolute size of the basic foot unit varied from place to place and from time to time.
After the 1791 founding of the Federation, one of the first tasks undertaken was standardizing weights and measures. Para's system had been more strongly influenced by Babylon's (multiples of six and sixty) than Rome's, and the only units in common were the one and sixfoot measures, the Paran foot being longer than the Irish. Federation delegates quickly agreed the basic unit would be the sixfoot fathom or staff, and the Parans also readily accepted the "decade" system already in use on Ortho. However, it took twenty years to agree on a common length for the foot, and only demand for a cooperative space program prevented the impasse from continuing.
In 1811 scholars pointed out the French on Prime were themselves engaged in a decading standards effort using a length they called the metre. Two metre amounted to one percent more than an Irish staff, and five percent less than a Paran rood (six of their feet), so both planets agreed to the equal inconvenience of an altered unit, and to keep things in harmony if units should later be changed on Prime. Thus a legal mile became a kilostaff, and the Parans employed the word, though almost no one used the term on Ortho. The centistaff (cs, but more commonly called "cent"), or hundredth part of a staff, became universal unit for small measure, replacing the longer inch. (Since an old Irish foot was 16.6cs, as opposed to the longer new (if it were used) of exactly a third of a metre, an old inch was 1.3833cs or 2.767cm. This must not be confused with the 2.54cm Prime inch which is still in use there.)
Thus the popular (but unofficial) unit of the yard or stick (half a staff, and socalled because it made a convenient length for a fighting stick) was exactly the same as a Prime metre. The obsolete term "sword" for this length is no longer used except by bladesmiths, who universally employ the new length in their craft, rather than the old swordyard of two Irish cubits or thirtyeight inches, easily permitting one to tell the era of a blade by its length. One also sees the occasional antique staff that has been lengthened by adding tips to conform to the modern unit. Many brehons still carry staves, but few people fight with them as it takes exceptional strength and agility to employ its length and weight effectively against smaller and faster weapons.
If we compare other units to those of Prime, the staff is now two of their metres exactly, the furlong 200, and the Irish mile 2000, or about 6562 English Earth Prime feet, as opposed to 6000 of ours (ironically, after being the cause of all the difficulties, the foot is obsolete today.) An acre (a furlong by ten staves) is still the thousandth part of a square mile, so it is 4000 square metres in Prime termsquite close to the size of their acre. Interestingly, because of differing conventions with respect to the inch, and the use of a mix of English and Irish rather than Gallic multipliers, the Prime Irish mile is longer still, at 6720 of their feet. A second irony is that the French here were the last to adopt the new system, even though their Prime counterparts invented the unit that allowed the Federation to agree in the first place.
Following this breakthrough, the decade commission quickly adopted the balance of the Prime metric system for all other measurement units, reasoning there was no hope of compromise on the vastly differing units the two planets had been using, and a third party system made more sense than further arguments. There have occasionally been proposals to adopt the Prime SI system entirely (that is, its length and area terminology as well), but the Federation names are too well entrenched now, and conversions, where necessary, are but by a factor of two, which is no great inconvenience even for scholars, and none at all for the general public. Besides, such an agreement could only have been reached in the first bloom of the exploration of Prime, when scholars were enamoured of nearly everything they found there. It would be impossible in today's more critical times.
from A History of Weights and Measures by Jana Whelan.