Q: I’ve read your article about why a “w” is called a “double-u.” What puzzles me is why we still have words with “uu”—i.e., “vacuum,” “continuum,” and “triduum.” And why the “w” in “weltanschauung” is pronounced like a “v.” Just curious.
A: As we said in that 2011 post, English words were written in runic letters until the seventh century, when the Latin alphabet was introduced.
But the Latin alphabet of that time had no symbol for the sound of “w,” so such a symbol had to be invented.
At first the symbol used was “uu” or “double u.” But in the eighth century the runic letter ƿ (called a “wyn”) was borrowed for this purpose and was used in English writing for several centuries.
In the meantime, the old “uu”, the Oxford English Dictionary says, “was carried from England to the continent.”
There, the OED explains, it was used to represent the “w” sound “in the German dialects, and in French proper names and other words of Germanic and Celtic origin.”
It was Norman scribes who introduced a ligatured version of the old “uu,” forming the letter “w.” This new letter traveled from France to England in the 11th century, and by about 1300 it had replaced the old rune ƿ in English writing.
Although this new “w” was probably regarded as a single letter from the beginning, “it has never lost its original name of ‘double U,’ ” the OED says.
Now for your question about why some English words continue to be written with “uu.” The reason is that they have retained the “uu” spellings they had in Latin.
For example, “vacuum” is from the Latin noun spelled the same way: vacuum.
However, the “uu” combination in English does not sound like “w.” In the case of “vacuum,” it can sound like “yoo” or like “yoo-uh.”
The latter pronunciation is a diphthong—two syllables merged into one sound. This double sound is observable in the spelling of the word in Italian, Spanish, and Portuguese: vacuo.
A handful of similar words spelled with “uu” also come from Latin words with identical spellings. And in these the “uu” is pronounced as a diphthong: “continuum,” “residuum,” and the uncommon “triduum,” meaning three days (or specifically the last three days of Lent).