A brief heads-up for the tailors out there.
I've been seeing some of what I consider to be inferior quality canvas cropping up and if you're not accustomed to looking for this, you may not notice it. So next time you are shopping canvas, bring a loupe or a magnifying glass.
The canvas is woven from yarns which are twisted; the animal hair will be mixed with wool, cotton, and maybe other fibers and twisted to make a yarn. The springiness of the animal hair, when twisted, will cause the yarn, and thus the canvas, to want to curl. To counter this, quality canvas is woven using what we call S and Z yarns (the shape of the letters gives you an idea of what this means); the yarn is twisted in one direction to create the S yarn, and the Z yarn is made by twisting in the opposite direction. This balances the canvas, but is more expensive than making, stocking, and using only one kind of yarn so certain weavers, mainly located in parts of the world which are new to weaving canvas (ahem) are cheating by only using one type of yarn. If you look at the photo above you can see a bit of the chevron which indicates that the weave is balanced.
If you're making up some heavy tweed or English flannel it probably won't matter, but if you're cutting something softer or lighter I would definitely take a closer look at the canvas you are buying.