Why were the princes barred from the throne?
Part of the controversy still surrounding Parliament's ruling that Edward (and his brother Richard) could not be rightful heirs to the throne arises from confusion about why Parliament ruled that their parents' marriage was invalid. There were two separate but related issues:
As a matter of law, the marriage was, indeed, invalid if the story of the precontract between their father and Lady Eleanor Talbot was true. Under both canon law and civil law, a "precontract of marriage" was a promise to marry, and it was enforceable in court as if the promised marriage had actually taken place. (The concept of a "precontract" still exists in law, but it usually arises today in the context of precontracting to make a contract for a business deal, like a sale of property or a corporate merger.) A precontract with Eleanor Talbot would have invalidated the king's marriage to Elizabeth Woodville. This was the law in England, and many other contemporary examples can be pointed to. The purpose of publishing the "banns of marriage", and then asking in the wedding ceremony if anyone knows of just cause why the marriage should not take place, was to prevent marriages that were invalid, because of a precontract or for any other reason. Marrying in "secret" (or "private", which usually meant "not in a church") wedding (without the calling of the banns, as Edward IV and Elizabeth Woodville did) was considered virtually an admission that there was a legal impediment. If Parliament was presented with evidence of Edward's marriage to Eleanor Talbot or his precontract to marry her, it was bound to rule that his marriage to Elizabeth Woodville was bigamous, and therefore any children born to them were bastards.
The fact that the princes were technically bastards (following his deposition from the throne, Edward V was referred to by his uncle's followers as the "Lord Bastard") did not necessarily mean they could never inherit -- William the Conqueror was neither the first nor the last bastard to inherit lands and titles. "Bastardy," the legal term for illegitimacy, was a legal status that could be changed by the law, either the law of the church or the law of the state -- as shown by the number of times King Henry VIII changed the status of his children. Parliament could have legitimized the princes and allowed Edward V to remain king, but it used that excuse for what it wanted to do for practical reasons. Boy kings (Henry III, Richard II, Henry VI) had always been disasters for England -- and the Wars of the Roses had been halted by the accession of Edward IV as a capable adult. The Yorkists were in power, and Edward V's numerous Woodville relatives had always been Lancastrians at heart and had already made many enemies. Richard III, on the other hand, was considered the Yorkists' best all-round candidate for the job of king at the time.
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