Q: Why has “naïve” survived, but not “coöperate”? Why do we write “Noël,” but not “poëm” or “reïgnite”? I’d appreciate (or appreciäte) any help you can offer on the rules for using the diaeresis. This particular issue is driving me dotty.

A: In a word like “appreciate,” the “i” and the “a” toward the end are clearly not a married couple.

Those two letters happen to be adjacent but they’re not a unit and aren’t pronounced as such. They belong to different syllables, and no one could mistake the way they’re sounded.

But in some words, two vowels side by side are pronounced as a diphthong—one vowel sound gliding into another within the same syllable, like the “oi” in “oil” or the “ou” in “loud.”

Then there are vowel pairs that might look like diphthongs but are in fact separate sounds in separate syllables.

In this case, a mark consisting of two dots over the second vowel can be used to show that the letter is sounded separately and not part of a diphthong.

This mark is called a “diaeresis” or “dieresis,” depending on which dictionary you follow. The two standard dictionaries we consult the most differ on this.

The American Heritage Dictionary of the English Language (4th ed.) prefers “dieresis” while Merriam-Webster’s Collegiate Dictionary (11th ed.) favors “diaeresis.”

Garner’s Modern American Usage (3d ed.) says the occurrences of “diaeresis” in print outnumber those of “dieresis” by three to one, which is why we’re going with the longer version here.

A classic example of the diaeresis is in the word “naïve,” where the first two vowels are phonetically divided: nye-EVE. The mark over the “i” tells the reader it’s pronounced separately.

A diaeresis is also placed over a lone vowel to show that it’s not silent, as in the name “Brontë.” But in most names (as well as words) with diaereses, the mark is suspended over the second of two vowels: “Chloë,” “Eloïse,” “Zoë,” “Noël.”

In practice, however, many familiar words are no longer written with diaereses, since readers already know how to pronounce them. In familiar names, the marks mostly serve as decoration.

As Fowler’s Modern English Usage (rev. 3rd ed.) explains: “Since the sign is not often on modern keyboards it is often omitted in printed work; and it has also usually been dropped from such familiar words as aërate, coöperate (now aerate, cooperate).”

But Fowler’s adds, “Occasional examples still occur, e.g., I reëntered the chestnut tunnel—New Yorker, 1987.”

Most publications don’t resort to diaereses as much as the New Yorker, where you’ll find spellings like “coördinate,” “reëngineer,” “preëminent,” “coöperative,” and so on.

In fact, “naive” often goes naked these days in publications other than the New Yorker.

In their entries for the word, both Merriam-Webster’s and American Heritage indicate that the diaeresis is optional. The spelling is given as “naive or naïve,” indicating that they’re equal variants and the choice is up to the reader.

You mention “poem” and “reignite.”

In fact, the first has sometimes been spelled with a diaeresis—“poëme” in the 1500s and “poëm” in the 1600s, according to the Oxford English Dictionary.

And we found a few examples in blogs of “reignite” spelled with a diaeresis, though the usage seems to be extremely rare and idiosyncratic.

Today when a writer worries that a word could be misread, the solution is usually a hyphen (“re-enter,” “re-ignite,” “co-op”), not a diaeresis.

The OED defines the noun “diaeresis” as “the division of one syllable into two, esp. by the separation of a diphthong into two simple vowels.”

It adds that this is also the word for “the sign [ ¨ ] marking such a division, or, more usually, placed over the second of two vowels which otherwise make a diphthong or single sound, to indicate that they are to be pronounced separately.”

The word for the mark was first recorded in English in 1611, according to citations in the OED.

It comes from the Latin diaeresis, but its source is the Greek diairesis (division), which in turn comes from the Greek verb diairein (to divide).

Now here’s a little detour.

The roots of that Greek verb are dia (apart) and another verb, hairein (to take or choose), which also gave us the word “heresy.” Etymologically, a “heresy” is a choice one makes, a “heretic” being one who makes the wrong choice.

But getting back to the diaeresis, don’t confuse it with its look-alike, the umlaut, which is also two dots above a vowel.

The word “umlaut” comes from German (um means “about” or “around” and laut means “sound”), and the mark is used in English only with German words and names.

It shows that a vowel sound has been modified, as in the word über or names like Göring and Gödel (which are sometimes rendered in English as Goering and Goedel).

Both the diaeresis and the umlaut are diacritical marks (or “diacritics”). They’re not punctuation; they’re phonetic guides. Such marks are becoming less common in English, though they cling to some foreign borrowings.

Besides the diaeresis and the umlaut, here are the most familiar diacritical marks, along with words they may appear with: the acute accent (“blasé”), the grave accent (“learnèd”), the circumflex (“bête noire”), the cedilla (“façade”), and the tilde (“señor”).

As for the “rules” on when and when not to use a diaeresis, the best authority is your dictionary.

Check out our books about the English language