What was the original feud between the Montagues and the Capulets about?
The word feud often denotes a disagreement that has gone on for so long that the original cause cannot be recalled by anyone. In historical and literary cases, it lends dramatic effect.
In Romeo and Juliet there are two important aspects to the feud betweent he "two households:" 1) no one can remember the original cause and 2) there are signs that the feud is cooling. Indeed, in the opening scenes when the Lords of both houses are getting ready to join the fray, the Ladies of both houses attempt to prevent them from doing so. Also, at the Capulets' Ball, Lord Capulet orders Tybalt to leave Romeo be as he is spoken of as a decent young man.
The tragedy of the ending is magnified because of the fact that the feud is cooling on its own. In fact, in my classroom, we have a discussion about whether this whole situation could have been avoided if the children had simply talked to their parents.
The feud between the Montague and Capulet families of Verona is, of course, central to "Romeo and Juliet." Romeo is a son of the Montague family while Juliet is a daughter of the Capulets. It is this feud which, ultimately, causes the death of the two title characters. The feud also embroils the servants of the families and even other citizens in the town.
Even though the feud is such a big deal, we are never told what it is about. It is presented in the Prologue as being "ancient" but beyond that, we don't find out anything about its causes.