Baron once meant simply the king's man, and its split created both houses of Parliament
Baron started out in Late Latin meaning a man, servant or soldier. After 1066 the Normans used it for anyone holding land directly from the king, and the way those men were summoned to royal councils, some singly and some as a group through the sheriff, laid the groundwork for the House of Lords and the House of Commons.
Etymologists have argued over the origin for centuries. In the 7th century Isidore of Seville connected it to a Greek word for heavy, after the hard labour of mercenaries, but the likelier source is Old Frankish, related to Old English beorn, meaning warrior or nobleman. The Normans carried the title into England in 1066 and then to Scotland and southern Italy, and it later spread to Scandinavian and Slavic lands.
In early Norman England a baron owed the king a set amount of military service each year and had to attend his council. Under Henry II a distinction appeared between greater barons, who held land by knight service, and lesser ones who held manors. Greater barons began receiving personal summonses to the council, a practice already visible with Thomas Becket in 1164, which grew into Parliament and eventually the House of Lords. Magna Carta in 1215 provided that the lesser barons of each county would be summoned together, sending elected representatives who became the knights of the shire, forerunners of the Commons.
Kings later created barons by writ of summons and eventually by letters patent, the only method used today, which cut the link to land. A series of statutes, including the Tenures Abolition Act 1660, left feudal baronies without legal force. When Britain introduced non-hereditary life peers in the 20th century, every appointee except the Duke of Edinburgh was given the rank of baron, now the lowest rung of the peerage below viscount.
Etiquette follows strict rules. A baron's coronet carries six equal pearls on a plain rim and is worn only at a coronation. He is addressed by the name of his barony, which is why Martin Rees is Lord Rees of Ludlow and why calling Margaret Thatcher Lady Margaret Thatcher would wrongly suggest she was a duke's or earl's daughter. British passports even record the title in the surname field.
Source: Baron
