Rationale behind pronunciation of "subtle"

Question

I've read pronunciation, yet I'm still irresolute about the exposition/logic behind the pronunciation of subtle. Why is the b not pronounced?

subtle = subtil(e) in French, in which the b is pronounced. That website avers:

"By the 18th century, however, they smartly dropped the silent b in dette, and doute, and decided to begin pronouncing it in subtil.

The deletion of b in these French nouns may explain the silent b in debt and doubt in English.
Yet why does this not extend to "su*b*tle"?

In French, dette and doute have no b (and thus b sound). Would this explain the silent b in "debt" and "doubt"?

In French, the b in subtil(e) is pronounced. Yet the b in "subtle" isn't? Why does this differ?

Answer 1

Is your question: why is the "b" pronounced in "subtil" in French? The reason is that people in France started pronouncing "subtil" the way it was spelled. People have a tendency to start pronouncing things the way they are spelled. This is why the English pronounce the "h" in "herb" and the "l" in "solder". The Americans still pronounce these words the way they were pronounced in the 1600s.

Is your question: why is the "b" pronounced in "subtle" in English? It's not.

Is your question: why was there a "b" in the spelling of "subtle" in English and French? It had lost the "b" in the pronunciation in both countries several centuries before. This is because there was a movement—I believe in the 16th and 17th centuries—in both France and England to put letters that had been in Latin back into the spelling of words. Before this, the words had been spelled the way they were pronounced. An example of this is the word "partial". The OED says that in Middle French and Middle English, it was most often spelled with a "c" (e.g., parcial, although there were many variants). This is because it was pronounced with an "s". Looking at the OED's citations, the spelling with a "t" starts becoming common in English the late 16th century. It's still pronounced with an "s" in France, and the English pronunciation has evolved to an "sh".

Answer 2

Firstly, the English pronunciation of "subtle" uses a silent "b":

subtle — /ˈsʌtl/ — fine or delicate in meaning or intent; difficult to perceive or understand

As for why we spell the word with a "b", this answer on Why does English spelling use silent letters? gives us a solid explanation:

[D]ebt comes via the Old French dete, which itself derives from classical Latin debitum. The b sound got lost due to French phonological rules/convention, and hence the French-origin pronunciation in English. Evidently, after the end of the Middle Ages in the 15th century, there was much revived interest in the classical world, and the spelling reverted to include the original b. Pronunciation, of course, stayed the same.

Presumably, the same explanation works for "subtle" and if we look at etymonline we see it noted:

Partially re-Latinized in spelling, and also by confusion with subtile.

Answer 3

Everywhere I've lived in North America (Texas, Alberta, Maryland, Washington DC, New York, and California), people have pronounced subtle in a way that rhymes with scuttle. It occurred to me, though, that some people might pronounce the now rare (in U.S. English) word subtile differently.

In checking Merriam-Webster's Eleventh Collegiate Dictionary (2003), I was surprised to discover that Merriam-Webster's does not consider subtile to be a variant of subtle; instead, it dates both terms to the 14th century, and provides each one with its own full entry and its own complement of definitions (many of them quite similar):

subtile 1 : SUBTLE, ELUSIVE [a subtile aroma] 2 a : CUNNING, CRAFTY b : SAGACIOUS, DISCERNING

subtle 1 a : DELICATE, ELUSIVE [a subtle fragrance] b : difficult to understand or distinguish : OBSCURE [subtle differences in sound] 2 a : PERCEPTIVE, REFINED [a writer's sharp and subtle moral sense] b : having or marked by keen insight and ability to penetrate deeply and thoroughly [a subtle scholar] 3 a : highly skillful : EXPERT [a subtle craftsman] b : cunningly made or contrived : INGENIOUS 4 : ARTFUL, CRAFTY [a subtle rogue] 5 : operating insidiously [subtle poisons]

With regard to your question about pronunciation, Merriam-Webster's offers only a no-b pronunciation of subtle, but it includes a voiced-b secondary pronunciation for subtile (the primary pronunciation it gives for subtile is identical to the one it gives for subtle). I have no idea how Merriam-Webster's arrived at its conclusion that a significant number of U.S. English speakers pronounce subtile with a voiced b.

In a Google Books search for works published between 1900 and 2000 that contain the word subtile, most turn up in the context of reprints or quotations from authors writing in previous centuries. The few truly modern exceptions chiefly involve a new word subtile (presumably pronounced sub-tyle) that refers to the underside of special heat-resistant tiles affixed to aircraft and spacecraft in aeronautics.