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Before the French Revolution, which started in 1789, French units of measurement were based on the Carolingian system, introduced by the first Holy Roman Emperor Charlemagne which in turn were based on ancient Roman measures. Charlemagne brought a consistent system of measures across the entire empire. However, after his death, the empire fragmented and many rulers introduced their own variants of the units of measure.
Some of Charlemagne's units of measure, such as the pied du Roi (the king's foot) remained virtually unchanged for about a thousand years, while others, such as the aune (ellused to measure cloth) and the livre (pound) varied dramatically from locality to locality. By the time of the revolution, the number of units of measure had grown to the extent that it was almost impossible to keep track of them.
Although in the prerevolutionary era (before 1795) France used a system and units of measure that had many of the characteristics of contemporary English units (or the later Imperial System of units), France still lacked a unified, countrywide system of measurement. Whereas in England the Magna Carta decreed that "there shall be one unit of measure throughout the realm", Charlemagne and successive kings had tried but failed to impose a unified system of measurement in France.^{[1]}
The names and relationships of many units of measure were adopted from Roman units of measure and much more were added  it has been estimated that there were seven or eight hundred different names for the various units of measure. Moreover, the quantity associated with each unit of measure differed from town to town and even from trade to trade to the extent that the lieue (league) could vary from 3.268 km in Beauce to 5.849 km in Provence. It has been estimated that on the eve of the Revolution a quarter of a million different units of measure were in use in France.^{[2]} Although certain standards, such as the pied du Roi (the King's foot) had a degree of preeminence and were used by savants, many traders chose to use their own measuring devices giving scope for fraud and hindering commerce and industry.^{[1]}
These definitions use the Paris definitions for the couture of Paris,^{[3]} and definitions for other Ancien régime civil jurisdictions varied, at times quite significantly.
The mediaeval royal units of length were based on the toise and in particular the toise de l'Écritoire, the distance between the fingertips of the outstretched arms of a man which was introduced in 790 AD by Charlemagne.^{[4]} The toise had 6 pieds (feet) each of 326.6 mm (12.86 in). In 1668 the reference standard was found to have been deformed and it was replaced by the toise du Châtelet which, to accommodate the deformation of the earlier standard, was 11 mm (0.55%) shorter.^{[5]} In 1747 this toise was replaced by a new toise of nearidentical length  the Toise du Pérou, custody of which was given to l'Académie des Sciences au Louvre.^{[6]}
Although the pouce (inch), pied (foot) and toise (fathom) were fairly consistent throughout most of prerevolutionary France, some areas had local variants of the toise. Other units of measure such as the aune (ell), the perche (perch or rood), the arpent and the lieue (league) had a number of variations, particularly the aune (which was used to measure cloth).^{[7]}
The loi du 19 frimaire an VIII (Law of 10 December 1799) states that one decimal metre is exactly 443.296 French lines, or 3 pieds 11.296 lignes de la "Toise du Pérou".^{[8]} Thus the French royal foot is exactly metres (about 0.3248 m).^{[9]}
In Quebec, the surveys in French units were converted using the relationship 1 pied (of the French variety, the same word being used for English feet as well) = 12.789 English inches.^{[10]} This makes the Quebec pied very slightly smaller (about 4 parts in one million) than the pied used in France.
Unit  Relative value (pieds) 
SI value 
Imperial value 
Notes  

point   ~0.188 mm  ~7.401 thou  of a ligne. This unit is usually called the Truchet point in English.  
ligne   ~2.256 mm  ~88.81 thou  of a pouce. This corresponds to the line, a traditional English unit.  
pouce   ~27.07 mm  ~1.066 in  of a pied du roi. This corresponds to the inch, a traditional English unit.  
pied du roi  1  ~32.48 cm  ~1.066 ft  Commonly abbreviated to pied, this corresponds to the foot, a traditional English unit. Known in English as the (properly a separate, shorter unit), the , or .  
toise  6  ~1.949 m  ~6.394 ft, or ~2.131 yd 
Six pieds du roi. This corresponds to the fathom, a traditional English unit. Unlike the fathom, it was used in both land and sea contexts.  
Paris  
perche d'arpent  22  ~7.146 m  ~7.815 yd  Related to, but not directly corresponding with, the English perch or rod (which is feet, approximately threequarters of the French perche).  
arpent  220  ~71.46 m  ~78.15 yd  Ten perches.  
lieue ancienne  10,000  ~3.248 km  ~2.018 miles  This is an old French league, defined as 10,000 (a myriad) pieds. It was the official league in parts of France until 1674.  
lieue de Paris  12,000  ~3.898 km  ~2.422 miles  This league was defined in 1674 as exactly 2,000 toises. After 1737, it was also called the "league of bridges and roads" (lieue des Ponts et des Chaussées).  
lieue des Postes  13,200  ~4.288 km  ~2.664 miles  This league is 2,200 toises or 60 arpents. It was created in 1737.  
lieue de 25 au degré  ~13,692  ~4.448 km  ~2.764 miles  Linked to the circumference of the Earth, with 25 lieues making up one degree of a great circle. (Compare the international nautical mile, of which 60 make one degree; one lieue therefore equaling 2.4 nautical miles.) It was measured by Picard in 1669 to be 2,282 toises.  
lieue tarifaire  14,400  ~4.678 km  ~2.907 miles  This league is 2,400 toises. It was created in 1737.  
North America  
perche du roi  18  ~5.847 m  ~6.394 yd  This perch was used in Quebec and Louisiana  
arpent (du roi)  180  ~58.47 m  ~63.94 yd  Ten perches du roi.  
Local  
perche ordinaire  20  ~6.497 m  ~7.105 yd  This perch was used locally.  
arpent (ordinaire)  200  ~64.97 m  ~71.05 yd  Ten perches ordinaires. 
Unit  Relative value (pieds carrés) 
SI value 
Imperial value 
Notes  

pied carré  1  ~1055 cm^{2}  ~1.136 sq ft  This is the French square foot.  
toise carrée  36  ~3.799 m^{2}  ~40.889 sq ft, or ~4.543 sq yd 
This is the French square fathom.  
Paris  
perche d'arpent carrée  484  ~51.07 m^{2}  ~61.08 sq yd  This was the main square perch in old French surveying. It is a square 22 pieds du roi on each side.  
vergée  12,100  ~1277 m^{2}  ~1527 sq yd  This is a square 5 perches on each side, or one quarter of an acre.  
acre, or arpent carré 
48,400  ~5107 m^{2}  ~6108 sq yd, or ~1.262 acres 
The French acre is a square 10 perches (one arpent) on each side. (Does not exactly correspond to the English acre, which is defined as 43,560 square feet.)  
North America  
perche du roi carrée  324  ~34.19 m^{2}  ~40.89 sq yd  This square perch was used in Quebec and Louisiana. It is a square 18 pieds du roi on each side.  
vergée (du roi)  8,100  ~854.7 m^{2}  ~1022 sq yd  This is a square 5 perches du roi on each side.  
acre (du roi), or arpent carré 
32,400  ~3419 m^{2}  ~4089 sq yd, or ~0.8448 acres 
This acre is a square 10 perches du roi on each side. Certain U.S. states have their own official definitions for the (square) arpent, which vary slightly from this value.  
Local  
perche (ordinaire) carrée  400  ~42.21 m^{2}  ~50.48 sq yd  This square perch was used locally. It is a square 20 pieds du roi on each side.  
vergée (ordinaire)  10,000  ~1055 m^{2}  ~1262 sq yd  This is a square 5 perches ordinaires on each side.  
acre (ordinaire), or arpent carré 
40,000  ~4221 m^{2}  ~5048 sq yd, or ~1.043 acres 
This acre is a square 10 perches ordinaires on each side. 
Unit  Relative value (pintes) 
SI value 
U.S. value 
Imperial value 
Notes 

roquille   ~29.75 ml  One quarter of a posson.  
posson   ~119 ml  
demiard   ~238 ml  ~0.5 pint  Etymologically, demi in French means "half": in this case, half a chopine, and  coincidentally  also approximately half a US pint.  
chopine   ~476.1 ml  ~1 pint  ~0.84 pint  
pinte  1  ~952.1 ml  ~2.01 pint  ~1.68 pint  Although etymologically related to the English unit pint, the French pint is about twice as large. It was the main small unit in common use, and measured of a cubic pied du roi. 
quade  2  ~1.904 L  ~0.5 gallon  ~0.42 gallon  
velte  8  ~7.617 L  ~2.01 gallon  ~1.68 gallon  
quartaut  72  ~68.55 L  A quartaut is 9 veltes, or two cubic pieds du roi.  
feuillette  144  ~137.1 L  
muid  288  ~274.2 L  The muid is defined as eight cubic pieds du roi.  
cubic  
pouce cube   ~19.84 ml  This is the French cubic inch.  
pied cube  36  ~34.28 L  This is the French cubic foot. In ancient times, a cubic foot was also known as an amphora when measuring liquid volume. 
Unit  Relative value (boisseaux) 
SI value 
imperial value 
U.S. value 
Notes 

litron   793.5 mL  0.1745 imp gal  0.1801 U.S. dry gal  of a quart. The litre is etymologically related to this unit. 
quart   3.174 L  0.698 imp gal  0.721 U.S. dry gal  of a boisseau. 
boisseau  1  12.7 L  2.8 imp gal  2.9 U.S. dry gal  Although etymologically related to the English unit bushel, the French bushel is about one third the size. A boisseau was defined as of a cubic pied du roi. 
minot  3  38.09 L  8.38 imp gal  8.65 U.S. dry gal  
mine  6  76.17 L  16.76 imp gal  17.29 U.S. dry gal  
setier  12  152.3 L  33.5 imp gal  34.6 U.S. dry gal  
muid  144  1,828 L  402 imp gal  415 U.S. dry gal  
cubic  
pouce cube   ~19.84 cm^{3}  ~1.211 cu in  This is the French cubic inch.  
pied cube   ~34.28 dm^{3}  ~2,092 cu in  This is the French cubic foot. Exactly 2.7 boisseaux. 
one quintal (100 livres) poids de table / poids de ville in  poids de marc equivalent (livres, approximations per source) 

Abbeville  93–94 
Avignon  83 
Beaucare  95 
Bordeaux  100 
BourgenBresse  96 
Dunkirk  87 
Lille  87–88 
Lyon  87 
Marseilles  81 
Montepellier  83 
Nancy  94–95 
Nantes  101–102 
la Rochelle  101–102 
Rouen (poids de vicomté)  103 
Strasbourg (petit poids)  96 
Toulouse  84 
nominal (marcs)  error in actual (grains) 

20  +1.4 
14  +4.5 
8  0.4 
4  2.1 
2  1.0 
1 (creux)  0.7 
1 (plein)  1.7 
Charlemagne's system had 12 onces (ounces) to the livre (pound).^{[13]} Between 1076 and 1093 King Philip I instituted a system of poids de marc (mark weight) used for minting coin, with 8 onces to a marc.^{[13]}
King Jean II constructed a new standard of measures, including a livre actuelle ("current" pound, also known as a livre de poids de marc or "mark weight" pound) of 2 marcs, i.e. 16 onces.^{[14]} The Charlemagne 12ounce livre became known as the livre esterlin ("true" pound) in order to distinguish it.^{[15]}^{[16]} "esterlin" was a Middle French word that did not survive into Modern French.^{[16]}
The livre actuelle could be subdivided into 2 demilivres (halfpounds), 4 quarterons, or 8 demiquarterons.^{[17]} Conversely, there were 100 livres in a quintal (c.f. English hundredweight).^{[17]} The fractional parts of an once had different names in Apothecary measure (used in medicine) and measure of precious metals, but the fractional ratios were themselves the same: 1 once was 8 drachme (Apothecary, c.f. English dram) or gros; 1 drachme/gros was 3 scruples (Apothecary, c.f. English scruple) or deniers, and 1 scruple/denier was 24 grains.^{[18]}^{[19]} This makes 384 deniers in a livre in weight measure, which contrasts with the old monetary livre in France which was divided into 240 deniers.^{[20]}
Jean II's standards are preserved in the Conservatoire Nationale des Arts et Métiers, which also holds a set of laterstill physical standards from the 15th century, the socalled pile de Charlemagne.^{[19]}^{[21]} This pile defined the weight of 50 marcs, i.e. 400 onces, and thus 25 livres actuelles, or 33⅓ livres esterlins.^{[13]}^{[22]} It had been kept in the royal palaces originally.^{[23]} In 1540 King François I had transferred it to the Cour des monnaies, where it had been held in a cabinet with three locks, whose keys had been held separately by the president of the Cour, one of its counsellors, and a clerk.^{[23]}
The thirteen individual pieces that made up the Parisian pile de Charlemagne comprised an outer containing cylinder nominally weighing 20 marcs, and a set of hollow nesting cups within, topped with a filled weight as the smallest piece.^{[22]}^{[24]}^{[25]} The heaviest cups were nominally 14, 8, 4, and 2 marcs, subtotalling 48 marcs; followed by a nominally 1 marc hollow cup which was termed the marc creux (hollow mark); and followed by 6 further cups (4, 2, and 1 onces, and 4, 2, and 1 gros) with a final seventh filled 1 gros weight, all totalling 1 marc, which was termed the marc plein (filled mark).^{[22]}^{[24]}^{[26]}^{[25]}
Unfortunately, the weights were not consistent, with the marc plein not being the same weight as the marc creux, and neither being the same as a mean 1 marc weight determined from the weight of the whole pile.^{[22]}^{[26]} So when the time came to work out the conversion factors between these measures and the metric system, the whole pile was taken to define 50 Parisian standard marcs, and thus 230400 grains (the number of grains in 50 marcs).^{[22]}Louis LefèvreGineau initially determined that the metric mass of the whole pile was 12.2279475kg,^{[26]} later corrected to 12.2376kg,^{[27]} thereby making (by division and rounded to three decimal places) a marc 244.753g, a livre esterlin 367.129g, and a livre actuelle 489.506g.^{[18]}^{[28]} Hence further the (Parisian) once was 30.594g, the gros/drachme was 3.824g, the denier/scruple was 1.274g, and the grain was 0.053g.^{[17]}^{[15]}
However, the actual masses of the premetric measures were nowhere near even this simple.^{[29]} These were just the Parisian standards, and individual provinces, cities, and even guilds, all had their own reference physical standards, which were not checked against one another and which sometimes conflated esterlin and actuelle.^{[29]} For just some examples: the Marseille livre was 399.6g, the Montpelier one 394.9g, the Toulon one 465.5g, and the Toulouse one 413.2g; with all of the fractional subdivisions having different values accordingly.^{[30]} The Limoges marc was 240.929g, the Tours one 237.869g, and the Troyes one 250.050g.^{[31]}
Furthermore, there were also livres comprising different numbers of onces to both the actuelle and esterlin, including livres of 14, 18, and 20 onces, confusing things yet further.^{[32]} The livre in the poids de table (table weight) systems used in Provence and Languedoc (and a common name for provincial weight systems in general alongside poids de pays, country weight, and poids de ville, town weight) was the same weight as 15 onces or even as low as 13 onces in the Parisian poids de marc,^{[33]}^{[34]}^{[35]}^{[11]} and the livre in the poids de soie (silk weight) system of Lyon was similarly just the weight of the Parisian livre.^{[36]}^{[11]} This caused an erroneous belief that these livres comprised 13, 14, or 15 onces, however this was a confusion stemming from the equivalent poids de marc weights, and both poids de table and poids de soie had 16 of their own, lighter, onces and so forth,^{[36]}^{[34]}^{[11]} Rouen had a poids de vicomté system.^{[11]}
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