Estates of the Realm

The estates of the realm, or three estates, were the broad orders of social hierarchy used in Christendom (Christian Europe) from the medieval period to early modern Europe. Different systems for dividing society members into estates developed and evolved over time.

Historically, in Northern and Eastern Europe, the Fourth Estate meant rural commoners.

The best known system is the French Ancien Régime (Old Regime), a three-estate system used until the French Revolution (1789–1799). Monarchy was for the king and the queen and this system was made up of clergy (the First Estate), nobles (the Second Estate), and peasants and bourgeoisie (the Third Estate).

In some regions, notably Scandinavia and Russia, burghers (the urban merchant class) and rural commoners were split into separate estates, creating a four-estate system with rural commoners ranking the lowest as the Fourth Estate. Furthermore, the non-landowning poor could be left outside the estates, leaving them without political rights.

In England, a two-estate system evolved that combined nobility and clergy into one lordly estate with "commons" as the second estate. This system produced the two houses of parliament, the House of Commons and the House of Lords. 

Whilst the estates were never [explicitly] formulated in a way that prevented social mobility, the English (subsequently the British) parliament was long based along the classic estate lines being composed on the "Lords Spiritual and Temporal, and Commons". The tradition where the Lords Spiritual and Temporal sat separately from the Commons began during the reign of Edward III in the 14th century.

In Scotland, the Three Estates were the Clergy (First Estate), Nobility (Second Estate), and Shire Commissioners, or "burghers" (Third Estate), representing the bourgeois, middle class, and lower class. The Estates made up a Scottish Parliament.

In southern Germany, a three-estate system of nobility (princes and high clergy), knights, and burghers was used.
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During the Middle Ages, advancing to different social classes was uncommon and difficult. 

"Medieval political speculation is imbued to the marrow with the idea of a structure of society based upon distinct orders," Johan Huizinga observes. The virtually synonymous terms estate and order designated a great variety of social realities, not at all limited to a class:
There are, first of all, the estates of the realm, but there are also the trades, the state of matrimony and that of virginity, the state of sin. At court, there are the 'four estates of the body and mouth': bread-masters, cup-bearers, carvers, and cooks. In the Church, there are sacerdotal orders and monastic orders. Finally there are the different orders of chivalry.
This static view of society was predicated on inherited positions. Commoners were universally considered the lowest order. The higher estates' necessary dependency on the commoners' production, however, often further divided the otherwise equal common people into burghers (also known as bourgeoisie) of the realm's cities and towns, and the peasants and serfs of the realm's surrounding lands and villages. A person's estate and position within it were usually inherited from the father and his occupation, similar to a caste within that system. In many regions and realms there also existed population groups born outside these specifically defined resident estates.

Legislative bodies or advisory bodies to a monarch were traditionally grouped along lines of these estates, with the monarch above all three estates. Meetings of the estates of the realm became early legislative and judicial parliaments. Monarchs often sought to legitimize their power by requiring oaths of fealty from the estates. Today, in most countries, the estates have lost all their legal privileges, and are mainly of historical interest. The nobility may be an exception, for instance due to legislation against false titles of nobility; similarly British government well maintains the distinction – witness its House of Lords, and the House of Commons.
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The medieval Church was an institution where social mobility was most likely up to a certain level (generally to that of vicar general or abbot/abbess for commoners). Typically, only nobility were appointed to the highest church positions (bishops, archbishops, heads of religious orders, etc.), although low nobility could aspire to the highest church positions. Since clergy could not marry, such mobility was theoretically limited to one generation. Nepotism was common in this period.

Another possible way to rise in social position was due to exceptional military or commercial success. Such families were rare and their rise to nobility required royal patronage at some point. However, because noble lines went extinct naturally, some ennoblements were necessary.
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Notwithstanding the House of Lords Act 1999, the British Parliament still recognises the existence of the three estates: the Commons in the House of Commons, the nobility (Lords Temporal) in the House of Lords, and the clergy in the form of the Church of England bishops also entitled to sit in the upper House as the Lords Spiritual.
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Today the terms three estates and estates of the realm may sometimes be re-interpreted to refer to the modern separation of powers in government into the legislature, administration, and the judiciary. Additionally the term fourth estate usually refers to forces outside the established power structure (evoking medieval three-estate systems), most commonly in reference to the independent press or media.