Why is Edward I of England given the numeration "I", when he was preceded by Edward the Confessor and Edward the Martyr. Did numerations only begin with William I?
Here's my suggestion for numbering the Anglo-Saxon kings Edward in future writing. Edward the Elder could be called "Edward the First or the Minus Second", Edward the Martyr could be called "Edward the Second or the Minus oneth", and edward the Cnfessor could be called "Edward the Third or the Zeroth". And there numerals can be written I/-2, II/-1, and III/0.
In the Middle Ages is wasn't common to use numbers for monarchs of the same name.
For example, I don't think that the eastern Roman or "Byzantine" emperors used numbers, and have read that Gibbon in The History of the Decline and Fall of the Roman Empire (1776-1789) was the first to number them.
In western Europe, the popes and the Holy Roman Emperors used numbers. The first Imperial example I know of was by Otto III in 1001.
Wikipedia's article on Pope John Numbering leads me to believe that popes started using numbers sometime between 983 and 1276.
It was very rare for kings to use numbers in the middle ages. The earliest example I know of is King William II of Sicily (1166-1189):
His numeral is contemporary and he himself used it.[a]
And numbering of kings remained rare and unusual until kingdsoms gradually started officially numbering all their kings about 1500-1800. Most numbers for kings, bishops, nobles, etc. were assigned by later historians centuries or millennia after they lived.
The idea that there was a rule to start numbering English kings with the Norman conquest in 1066 is inaccurate. Such an idea would be based on the idea that England really began with the Norman conquest, and is completely inconsistent with the opposite and equally inaccurate idea that all that is good in England comes from Anglo-Saxon England before the conquest.
Willliam the Conqueror and his followers and heirs had absolutely no reason to say that Edward the Confessor and his predecessors were not kings of England. William claimed that he was the rightful and legitimate heir and successor of Edward the Confessor.
But the numbering of English Kings didn't start until centuries later. Thus the creators of the numbers didn't share William's stronge deisire to prove continuity with the kings before the Norman Conquest. It wasn't very important to them how much continuity there was with the Anglo-Saxon kings of England.
Three English kings in a row were named Edward, becoming king in 1272, 1307, and 1327 respectively. Legal documents, such as might be cited in lawsuits, were dated by the years of the monarchs reign. So in the time of the third Edward, a ducument dated to the third year of king Edward might date to 1282, 1317, or 1337.
So people got in the habit of informally giving numbers to those three Edwards so they could immediately realize the Anno Domini dates of legal doucuments and reduce the confusion.
At some point, though, people seem to have started switching to numbers. The fact three Edwards ruled in turn between 1272 and 1377 meant that, even in some chronicles published in his own lifetime, the last of these was referred to as Edward III. (A matter, one assumes, of convenience.) These numbers stuck, when a standard numbering began to appear some time in the 16th century - though whether this was because they were already in use, or because the Tudor chroniclers consciously decided to start counting at 1066, is not exactly clear.
This article claims - though I doubt its accuacy - that Edward I wanted to be numbered Edward IV:
Longshanks is officially called Edward I, but he originally planned for a different name. Even though he was the first Edward since the practice of numbering English kings began, Longshanks wanted to be called Edward IV, a nod to the Anglo-Saxon kings who had ruled before him. For unknown reasons, however, he was outvoted and ended up going as Edward I.
And the numbering used in the tiem of Edward III might have been continued in the reign of Edward IV (1461-1483).
A document describes Richard, Duke of York (1473-1483?) as the second son of King Edward the fourth.
Henry VIII (r. 1509-1547) officially described himself as Henry the Eighth in his official documents, the first English king to do so consistently.
[The movie Becket (1964) has a cute scene where King Henry II is annoyed by his young sons play fighting in anachronistic plate armor. Henry stops one of the boys and asks him which one he is. The boy says "Henry the Third!" and Henry II growls "Not yet!". But using regnal numbers in the time of Henry II was totally anachronistic.]
So when Henry VIII's son Edward became king in 1547, that was the time to decide how the Kings Edward would be officially numbered. And someone chose to number Edward as "the Sixth", thus ignoring Edward the Elder, Edward the Martyr, and Edward the Confessor for the purpose of numbering English kings named Edward.
So what were the names of the pre conquest Kings of the Anglo-Saxons and of the English?
Alfred, Edward (3), Aelfweard, Aethelstan, Edmund (2), Eadred, Eadwig, Edgar (2), Aethelred, Sweyn, Canute, Harold (2), and Harthacnut.
I note that some modern historians give numbers to some ofthose Anglo-Saxon Kings.
To make matters worse, at least if you're a slightly anal history nerd like me, some later writers decided to impose numbering on the other kings whose names appear more than once before the Conquest. There's an Edmund I (939-946) and an Edmund II (seven months in 1016); a Harold I (1035-1040) and a Harold II (10 months in 1066; second time was not, it seems, lucky). The result of this is that we have a standardised numbering scheme on either side of the conquest, except for the one minor point that there have been 11 kings of England called Edward and we only number eight of them.
Alfred, Edward, Edmund, Edgar, and Harold are still commonly used as English names, and possibly by members of the royal family in recent centuries.
But immediately after the conquest members of the royal family, including kings, used names which hadn't been used by the House of Wessex of Anglo-Saxon England: William, Robert, Henry, Geoffrey, Richard, John. Those names have been repeatedly used down the centuries in the English royal familes with the additions of James, Charles, and George.
So there are kings numbered as high as William IV, Henry VIII, Richard III, James II, Charles II, and George VI.
The first Anglo-Saxon king's name to be used by the post-conquest royal Family was Edmund, as in Edmund II, Earl of Cornwall (1249-1300), and Edmund "Crouchback", Earl of Lancaster (1245-1296), both grandsons of King John.
The only Anglo-Saxon royal name which has ever been used by a later king of England is Edward, and I have explained why I think the Edwards are numbered the way they are, the Duke of Windsor numbered as Edward VIII instead of Edward X or Edward XI.
And the English kings Edward is not the only case of mixed up, inaccurate, or illogical numbering of monarchs.
Yes, the kings of England are traditionally numbered post-conquest. Additionally, monarchs who held England and another country in personal union are often given both numbers, like James VI/I, James II/VII, less commonly Henry VI/II, and (more controversially, as he didn't hold both kingdoms at the same time) Louis I/VIII. Order of the numbers is primarily determined by the order in which he became king of each, not lower first.