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The destruction of the Roman Empire in 476 AD left Europe in a vacuum in many ways. Although religion remained fairly stable, many other aspects of European culture that had relied on the Empire fell apart. The power structure consisting of Emperor, governors, Imperial bureaucrats and the Roman army in an instant failed to exist. It would take centuries for a new structure to arise more complicated than simply the barbarian1 kings and their followers. This structure is the nobility, which developed in roughly the same ways across Europe, and which marks for many the true beginnings of the medieval period. In European medieval society, kings held supreme power within their kingdoms2 but their power tended to be severely limited both by powerful members of the clergy, who tended to control the money, and the nobles who supplied most of the king's army. An overview follows of the various ranks, tiers and stations of authority in a typical medieval kingdom.
Obviously not nobility, the commoners were the bottom of the heap in feudal society, although by far the largest group. Commoners included peasants and serfs (the farmers who tilled the lands), thieves, bandits and other criminals, and merchants, as well as most artists and craftsmen in the towns. A key indicator of commoner status was that commoners generally were forbidden to bear arms. They had essentially no rights and could be controlled by law in any way. A major exception to the general oppression of commoners were the Freemasons who, as the most essential and organized of craftsmen, were granted special privileges in travel and working rights. In England, the polite and proper way to address a commoner was 'goodman' or 'goodwoman', leading to the much later 16th- and 17th-Century term 'goodwife'.
The main distinguishing characteristic of a gentleman was that he did not work for a wage. In other words, gentlemen were non-landed3 individuals without a trade or occupation but with an income. In practice, this generally meant that they owned land, but this was a private matter and not official rule or governance of that land, which would most likely be ruled by a landed noble. One could also be a gentleman by the support of a stipend from a higher-ranking noble in return for services such as being a scribe, astrologer, or someone with another sort of academic skill. Doctors were also gentlemen, and were careful not to be considered professionals. 'Doctor' was originally an academic title that simply means 'teacher', and the use of the term indicates that they saw themselves as academics rather than professionals, unlike surgeons, who performed actual surgery and were commoners.
Esquires were originally the personal servants, apprentices and attendants of knights4. The use of the term by barristers in the UK and and all lawyers in the United States is somewhat informal and is intended to demonstrate that lawyers are above simple gentlemen. The fees paid for a barrister's appearance in court were carefully distinguished from wages and were considered to be a reparation for the indignity that a barrister underwent by associating himself with his client. Esquires are permitted to append the word 'esquire' to the end of their name.
The knight was the basic fighting unit of any medieval army. A knight swore allegiance to his lord, who could be of any greater rank, and was required to serve in battle on his behalf. In general, knights were understood to be at least of great enough means that they could provide their own horse, sword, shield and armour, although there are occasions on which hastily recruited knights would be given supplies from the stockpiles of the lord himself. Knights might or might not own land, but in either case it was a private matter unrelated to the rank of the knight.
All knights had the right to be addressed as 'sir', usually followed by both their first and last names. The female equivalent of this term is 'dame'. The range in authority, wealth and prestige of knights is astonishing. They ranged from poor 'country knights' who often had to garden to provide their own food, to the almost completely autonomous knights in the southern part of the Holy Roman Empire5, who essentially ruled their own tiny kingdoms. In addition, knights could belong to chivalric orders, another way to gain autonomy from the lord: the largest and most powerful orders such as the Teutonic Knights, the Knights Templar and the Knights Hospitalier were sovereign entities in their own rights. Knighthood was not hereditary, but any knight could confer the honour on any other man.
The title 'baronet' did not date from the medieval period, but was created in 1611 by James I as a new product to sell in order to raise money. As such, there are no equivalent titles in other European nations. A baronet is addressed in the same way as a knight but can pass the title to his sons. In effect, this is a slightly more prestigious, inheritable knighthood.
Nobility and Royalty
Baron is the lowest rank of landed nobility, or in England the lowest rank of peerage6. The term comes from Old German baro. A baron rules prescribed lands within a county, but there is no official term for that land. Although barons outside England are referred to by the title and their last name - eg, 'Baron Rothschild' - within England the title 'lord' is more common, as in 'Lord Byron'. To complicate matters, the term 'the Barons' is also used to refer to all ranks from baron up to and including duke, to be contrasted with 'the Princes', those above dukes. Barons, and the superior ranks of viscount and earl, are referred to as 'Right Honourable'.
An English variant on the system is the creation of a 'life peerage'. A life peer is given all the same rights and respect as any other, but the title is not inherited. Over history, most life peerages have been baronies, and the person is thus called Lord (or Lady) Smith, unless there is already a Lord Smith, in which case it is Lord Smith of Wherever. Children of life peers gain no title, but are properly referred to as 'The Honorable' within their lifetimes.
Viscount comes from the Latin word vicecomes meaning 'vice count'. As with barons, a viscount is addressed by title and last name - eg, 'Viscount Palmerston' - and there is no term for the tract of land ruled by a viscount.
The word 'count' comes from the Latin comes, meaning 'companion'. In the Roman Empire, a count was the personal retainer of the Emperor who would be granted authority over a diocese or an important province. In the Middle Ages, such ruled areas came to be called 'counties'. In Germany, a count was a graf except in certain outlying areas where another title, landgraf or landgrave was created. Counts are referred to by title and area of authority: the 'Count of Toulouse' would be the authority over the 'County of Toulouse', at least in theory.
In England, the word count is only applied to counts outside of England, and English counts use the title 'Earl', from the Old English eorl meaning 'warrior'. The wife of an earl, however, still uses the title 'Countess'.
A county on the border with a particularly dangerous neighbour was referred to as a 'march' and its ruler as a 'marquess' in English or a 'marquis' in French. The most common term, however, is 'margrave', which is an anglicization of the German word markgraf. Prussia and Austria both started out as marches. The wife of a marquess is a marchioness. Marquesses are addressed in the same form as counts, as the 'Marquess of Wherever'. Traditionally, a marquess had far more authority than a count, being able to collect taxes and administer justice directly over their subjects. Margraves are referred to as 'Most Honourable'. In more modern peerages, marches are called 'margravates'.
The old Roman title dux meant 'military leader', so that in Roman times there was both a comes and a dux of Britain. Because of his direct connection to the Emperor, the comes generally had more authority. This changed when the title of dux was used by subject tribal leaders in the post-Roman kingdoms. The use of the title spread, and it was generally used by semi-autonomous tribes or other areas seeking to establish a uniqueness from the general authority of the king. The area ruled by a duke is called a 'duchy' and the wife of a duke is a 'duchess'. In eastern Europe there was a comparative rank of voivode.
The use of 'duke' in England dates only from 1337, and English duchies were never anywhere near as large or as autonomous as on the continent. The title was often used without accompanying actual land. This is the case with the title of 'Duke of York' given to the brother of the Prince of Wales. It gives no actual authority over Yorkshire, which is a county. A Duke is referred to as 'Most Noble' or 'His Grace'. A varient of duke was the medieval title doge, for the ruler of independent cities such as Genoa and Venice.
The title Grand Duke was never used by anyone during the Middle Ages, and is a curious product of translation. The actual title - used for rulers in Kiev, Vladimir, and Lithuania - is knyaz in Russian, kunigaikshtis in Lithuanian, knize in Czech, etc. In more modern times, the brother of the Russian czar was referred to by the title Velikii Knyaz, which was always translated 'Grand Duke' by analogy with the British and French traditions of using 'duke' for the brother of the king. Following this example, the kynaz of Kiev and so on were also called Grand Dukes. Luxembourg today is an independent Grand Duchy.
Again, never actually used in medieval times, the title Archduke referred to the Hapsburg rulers of Austria after the country was unilaterally upgraded from a duchy in 1457 and before it was upgraded to an empire, although the actual title used was Erzherzog. After the declaration of the Austrian Empire in 1812, the title was used by the heir to the throne of Austria, including Archduke Franz Ferdinand. There was only ever one archduchy.
The term 'prince' derives from the Latin princeps, or 'first citizen', which was one of the titles bestowed on Roman Emperors and was a particular favourite of the first Emperor, Augustus. 'Prince' and 'princess' are the traditional terms for the children of the king, although the term was also used for children of independent dukes. Children of lesser rulers were called 'lord' or 'lady'. Generally, the title of prince is accompanied by a theoretically associated tract of land - a principality - but actual direct rule of a principality by a prince or princess is rare. Although technically of higher rank than a count or duke, independent rulers rarely used the title of prince because it tended to be associated with children. Nevertheless, there are still sovereign princes of Monaco and Liechtenstein.
The word 'king' derives from the German konig, meaning the leader of the kin: the tribe. The idea of kingship was something that the Romans vehemently avoided, which is why no titles derived from the Latin term rex have come down to us7. On the other hand, both rex and the related Gothic term reiks are derived from the Indo-European root rejks, which also gives us the term for the Indian raj and the German name for a kingdom: reich.
Despite the great variety of political systems in the Middle Ages, most political structures were headed by a king who ruled a kingdom. The king was technically sovereign although, in reality, the power of the king depended greatly upon his ability to assure the loyalty of his dukes. Because the feudal structure involved both giving most of the land to lower-ranking nobles and putting intermediaries between the king and the knights who actually fought, kings had direct command over neither wealth nor soldiers. They had to depend on their ability to manipulate everyone else to stay on top. Depending on the prestige of their particular reign, kings may have been addressed by titles such as 'Your Highness' or 'Your Majesty', but then again they may not.
In the feudal world, emperors were a definite anomaly, the rarest of titles other than the aberration 'archduke'. In the Roman world there was only one Emperor, and that term was never really considered to be the prime title of the head of the Empire. Depending on the Emperor, they often used the title Augustus ('great one') or Caesar8. As mentioned, Augustus himself was fond of the term princeps. Imperator actually meant nothing more than 'commander', although more specifically it was the title bestowed upon an extraordinarily successful Roman commander. When the Empire itself fell, it was still generally considered that there was only one emperor, although which emperor it was depended on where one lived: in the lands under the authority of Constantinople it was the Byzantine Emperor, while in the lands controlled by Charlemagne and his successors it was the Holy Roman Emperor, who was supported by the authority of the Pope. Either way, there was only ever one and nothing more would be tolerated because the Emperor was considered to be the only legitimate successor of the Roman Empire.
It was only in the borderlands that recognized the authority of neither empire that other emperors cropped up, and that only very rarely. After the fall of Constantinople to the Ottomans in 1453, the imperial claim again fissioned, so that the Holy Roman Empire, Ottoman Empire and Russian Empire all claimed to be the legitimate successor to Rome. It wasn't until the rise of Napoleon that the title really got diluted, but before long all sorts of upstarts were claiming an emperor: Trebizond, Ethiopia and Britain.
A good overview of English peerage can be found at hereditarytitles.com.