Many a textbook will confidently tell you, "In the Middle Ages, society was divided into three orders." If you've been reading this blog, you will have long ago figured out that a lot of what the textbooks tell you is completely wrong, either based on the assumption that people were simpler (and stupider) in the Olden Days, or else derived from a radical simplification of history--or often both.
So it will come as no surprise to faithful readers that "the three orders" did not exist! Is this going to be an extremely short blogpost, you ask? Well, there's actually more to it.
Humans like to categorize. Everyone "knows" that modern American society is divided into upper class, middle class, and working class, even though pretty much everybody will tell you they are middle class, and the borders between the classes (if any) are hard to define.
In the same way, medieval thinkers would periodically come out with grand schemas to categorize their society. A popular schema was so-called secular clergy (bishops and priests), monks and nuns, and laypeople (that is, people not in the church). Another was the virgins, the unchaste, and the married. These were all based on ideas of "three." Three is a good number. That is why we have "three classes" in the US. That is why the three bears' porridge was either too hot, too cold, or just right.
Around the beginning of the eleventh century, a bishop came up with a categorization that may seem closer to what the textbook meant by "three orders." He said that society was divided into "those who pray, those who fight, and those who work," and that each of these groups should stay in their proper place. He was addressing the then French king, telling him that the monks who had been advising the king, "those who pray," should stay in their own place, not interfere with "those who fight," which included the king. Interestingly, this bishop, by his own definition, was one of "those who pray," but that, of course, was totally different (in his own eyes). He should advise the king.
This definition, problematic from the beginning, got little attention. And in fact it was a very inaccurate characterization of society. The "workers" would have to include everyone from serfs up through wealthy merchants and artisans. "Those who pray" would have to include everyone in the church, monks, nuns, bishops, priests, and university professors, though these were often at odds with each other. The "fighters" would have to include everyone from kings to service knights—and where are the noble women supposed to fit in?
In practice, young men from peasant backgrounds constituted the first, eleventh-century knights. Wealthy townsmen would educate their children as aristocrats. Church leaders and leaders of secular society (castellans, duke and counts) came from the same families. Nothing kept the ambitious from trying to be socially mobile—though whether they succeeded or not was a different question, as it is in the modern US.
The supposed "three orders" only became semi-official at the beginning of the fourteenth century, with the establishment of the Estates General in France. This was a council that was supposed to advise the king, made up of representatives of the church, the nobility (including knights), and "everybody else," basically wealthy townsmen (no peasants need apply). In practice the Estates General rarely met, and its greatest moment was starting the French Revolution in 1789.
Interestingly England's Parliament, a generation older than the French Estates General and much more influential, had (and has) two chambers, not three. Church leaders (bishops and abbots) were grouped with the Lords (greatest nobles), while the Commons included both knights and wealthy townsmen.