Ever wear an Icebreaker or Smartwool baselayer? That merino wool you love actually has its origins in medieval Spain. Lots of places in western Europe did indeed produce their own wool.

We need to be careful, though, to distinguish raw wool from woolen cloth. Everywhere grazed their own sheep and everywhere had their spinners and weavers and dyers. But population growth, urbanization, technological developments, and international trade did indeed make some places centers for fabric production, and some places--namely, England and Spain--sources for wool itself.

One factor often pointed to is wool quality. "English sheep had the finest wool." And yes, England did have a reputation for producing wool that could be turned into the best-quality cloth. This was especially important for the export-oriented textile industry of the Low Countries, which depended on producing high-quality fabrics for Mediterranean markets.

But as John Munro has pointed out several times, not every sheep from England--not the majority of them, in fact--was producing luxury-caliber wool, and sheep in other places were. So quality alone can't explain the dominance of England (and again, Spain) in wool production.

The thing about grazing sheep, though, is that it takes land. And land being roamed by sheep is land that can't be used for growing food.

So cities like Paris, Bruges, Ghent had people to feed and clothe. And as population grew, not only did more land around the cities need to be dedicated to growing above all grain. Also, there were major problems with over-grazing sheep.

Meanwhile, English monks and nuns, and Spanish farmers were finding that pasturing or transhumance were much more efficient uses of their land than ekeing out grain growth.

With England in particular, its wool export trade became closely tied to Low Countries' woolen cloth production, which tended to spiral upwards together and reinforce each other's dominance of either raw commodities or manufactured goods.

Of course, things could get very, very interesting. England would later make things complicated with hefty export taxes as its monarchs finally figured out how to exploit the wool trade to fund their armies without making their own population grumpy.

Oh, and then there's the trade war launched by the countess of Flanders and Hainaut over wool in 1275. Yes, a trade war. Because the Middle Ages are the best ages.