When reading documents about land our ancestors owned, we often see references made to a certain number of arpents under cultivation or not under cultivation, especially in census records.
An arpent is a measurement and has nothing to do with the condition of land. An arpent is about 192 feet in length but is also used to specify a quantity of land such as an acre. I believe the quantity of an arpent would be a square arpent but the word square is not used.
In the early days of New France everyone wanted and needed land with river frontage. So most lots were 2 or 3 arpents of frontage. These lots went back 20 to 40 arpents.
If you drive from Québec City to Ste-Anne or around the Isle d'Orléans, you can see the fence lines that still conform to those measurements. In many cases descendants of the original owners are still on the property.
Old French measurements
Linear
12 linge = 1 Pouce
18 Pied = 1 Perche
12 Pouce = 1 Pied
180 Pied = 1 Arpent
6 Pied = 1 Toise
A lieue(League) = 3.1 miles
Area
Arpent Carrée = 0.846 Acres This was usually simply called an Arpent so you must know the context in which arpent was used.
French-English equivalents
French English
Pied 1.066 feet
Toise 6.396 feet
Arpent 191.88 feet
A man said to be 5 feet 4 inches tall by early French measurements would actually be 5 feet 8 1/4 inches tall by our measurements. An easy way to convert is to change the height to inches, multiply by 1.066, then change back to feet and inches.
Most of the above is from "Our French-Canadian Ancestors" by Laforest. The table is in Vol 1 Appendix B1 and Vol 22 page 273(Same as Vol 1) Also in Vol 1 Appendix D2,3,4 are maps
of the property lines through Chateau-Richer, L'Ange Gardien, and Ste-Anne in 1680. I'm sure you have some ancestors there.
This information courtesy of Ray Roux of Mexico, New York.