I’ve heard it said that using II implies that II was not the son of I.
If John Jacob Smith’s son is John Jacob Smith, then the son is Jr.
If John Jacob Smith’s son is John Edward Smith, and John Edward Smith’s son is John Jacob Smith, then the grandson is John Jacob Smith II.
Therefore, if someone were named John Jacob Smith II you would know that there was some sort of break between him and John Jacob Smith I.
Of course, the great thing about standards is that there are so many to choose from…
That should read:
The question of what happens to Mrs. John Smith Sr. when her son dies and her grandson succeed to the title of plain John Smith is not addressed.
Regarding the upper limit of the numbers, I recall reading around 1992 of a 500th anniversary event. A featured speaker was “Christopher Columbus XX”.
For the record, Miss Manners is giving one standard usage in etiquette – how to address another person who, e.g., has the same name as both his late father and living son.
I’m giving the traditional historical usage – John D. Rockefeller IV is using that name because people do refer in writing to his grandfather and great-grandfather as Sr. and Jr., even though both are dead and he’s the only living JDR.
And if anyone asked you when John F. Kennedy died, most people would say 1963, not 1997 or whenever it was – even though the late President’s son was the only living John F. Kennedy in his lineage, he was still JFK Jr. to the world.
No, sorry. My father is William Thomas III, even though his grandfather, William Thomas Sr. has been dead for decades.
Here is a summary of what Miss Manners actually says about it (I think this is from the Guide for the Turn of the Millennium):
In terms of traditional etiquette:
The oldest living bearer of the name (say, John William Smith) uses no suffix.
John William Smith’s identically-named son is John William Smith, Jr., whose identically-named son is John William Smith III.
When grandpa John William Smith dies, everybody “moves up” a notch: the current “Jr.” loses his suffix, and “III” becomes “Jr.” Yes, this is bloody confusing, but you have to do it, unless you’re royalty or some other kind of multigenerational public figures where there’s some point to keeping the numbers consecutive.
The late John William Smith’s widow uses “Sr.”, as in “Mrs. John William Smith, Sr.”
An identically-named male not in the direct line of descent uses “II” rather than “Jr.”: so if the original (or most recent) John William Smith has a nephew whose parents give him the same name, the nephew goes by “John William Smith II”.
In terms of real life:
Personal preference trumps traditional etiquette in this case. If someone gets used to being known as “John William Smith Jr.” and doesn’t want to change it even when he inherits oldest-living-representative status, that’s his choice. Nobody is allowed to tell him he’s using the wrong suffix.
ecg: If John Jacob Smith’s son is John Edward Smith, and John Edward Smith’s son is John Jacob Smith, then the grandson is John Jacob Smith II.
Oh yeah, I forgot about that case. A “II” can be in the direct line of descent as long as he’s not an immediate descendant. Only an identically-named son gets the “Jr.” suffix.
I’m surprised nobody’s brought up John Adams and John Quincy Adams, both former US Presidents.
As to reasoning, I’ve no clue, and the answers seem fine, but it’s certainly not unprecidented. Whatever gripes I have with Dubya, his use of the middle initial isn’t among them.
That’s fine, but this isn’t IMHO. The question is what the upper limit is on numerals suffixed to names. According to traditional social practice, the limit is the number of currently living generations. That this is a good and wise custom is Judith Martin’s opinion, and you may disagree, but that is not the purpose of my citing her. It is to address the factual question of what the upper limit is. Obviously, people can name their children whatever they want and adopt whatever personal styling they want, so the only reasonable answer to the OP’s question is what etiquette and tradition dictate (which may or may not be what is commonly practiced.) If you believe she’s wrong about this, do you have a cite? (In fairness, fezpp linked to a Wikipedia entry that claims that their is no standard in tradition or etiquette, but I’d like to know if any actual experts on etiquette and social custom actually disagree with Miss Manners.)
Miss Manners never said anything of the sort. It would be entirely at odds with her usual manner, as BrotherCadfael pointed out. She did say that nobility and other public figures are exempt from the rule of “moving up” when someone dies.
Not even Joe Sixpack VI? 
As Alan and BC have pointed out, Miss Manners certainly doesn’t prohibit Joe Sixpack or any other non-royals from using these suffixes. If Joseph Case Sixpack wants to call his son Joseph Case Sixpack, little Joe is just as much entitled to the suffix “Jr.” as the son of Reginald Plantaganet Crystal-Decanter is.
And if Joe Jr. wants to pass on his name to his son while the original Joe is still around, the new model becomes Joseph Case Sixpack III. According to etiquette, they have to move up when the original Joe kicks the bucket, so the existence of Joe Sixpack VI would technically imply that old Joe had hung around long enough to see his great-great-great-grandson. However, even if Joe VI is not in fact the sixth living bearer of that name, Miss Manners is not going to brutally take his cherished suffix away from him.
This provoked another question in my mind: what happens to the “moving up” process when mortality skips a generation? That is, if Joe Jr. succumbs to a tragic hunting accident while old Joe is still alive, does little Joe III now become “Jr.”? I would guess not, and that the “promotion” wouldn’t happen until old Joe goes. But does anybody know for sure?
Bill Clinton’s surname at birth was Blythe.
In genealogys he is usually referred to as “William Jefferson Blythe IV”.
This would seem to imply there was a “William Jefferson Blythe III”(his father, yes there was),a “William Jefferson Blythe II”(his grandfather, yes there was) and Bill’s Great grandfather William Jefferson Blythe.
Whoops, his name is supposed have been Thomas Jefferson Blythe. Try as you might, you can’t find anyone in Bill’s family named William Jefferson Blythe before his grandfather.
I don’t remember which book I saw it in ( I 'm pretty sure it ws Miss Manners), but Joe III does not become “Jr”. He becomes “II” , just as he would have been if there had never been a “Jr”.
Just to add another data point, a classmate of mine has the same first and last name as his father, grandfather, and great-grandfather, and has given his son the same first and last name. All of them have different middle names (in fact, he typically calls his son by his middle name to distinguish them from the rest), but he still formerly refers to himself and his kin with the roman numeral included.
As for the Miss Manners convention of changing the numerals, how does one refer to a now-deceased member of the family? “Ah, yes, I used to know Joe Sixpack who was the III, until he became the II, and who is now just plain Joe Sixpack, being dead.”.
Chronos: As for the Miss Manners convention of changing the numerals, how does one refer to a now-deceased member of the family? “Ah, yes, I used to know Joe Sixpack who was the III, until he became the II, and who is now just plain Joe Sixpack, being dead.”
I guess you’d use kinship or biographical information, as in “I used to know Joe Sixpack, the grandfather of the current Joe Sixpack” or “I used to know Joe Sixpack, the one who won three silver cups at the Hog Pond Gymkhana back in 1959.”
I’d say the advent of modern computerized record-keeping and identification spells the death of the “moving up” rule. If a chap is born “Joseph Sixpack IV,” then a multitude of databases is going to keep him “Joseph Sixpack IV” unless he takes positive action to change that, regardless of who does when.
Hmmph. My family is somewhere between Sixpack and Bush status but certainly not royal. Grandpa, RIP, was Raymond Francis C. Dad was baptized Raymond Fayette C., but informally changed his middle name to Francis when he was 20 or so. My baby brother is Raymond Francis C. III, not Junior. Nobody moved up when Grandpa died, which was a lot of fun when mail arrived for Dad addressed to Jr. and he opened letters–including once steamy one from a girlfriend–that were meant for baby bro. 
Baby bro got married and is having a son they’re going to call Jacob Alexander, so unless they have another boy, no more Raymonds. Both men only uses their suffixes in correspondence and Raymond III used his on his wedding invititations and stuff, but IRL Ray’s the Dad and Raymond’s the son, unless Raymond’s among his friends, who call him Crazy bleepin Ray.
Glad I’m a girl. Now, my mother actually has the identical name to her mother except for the addition of a middle name…but that’s another thread.