Why are there some inert letters?

Question

My original question was: why is ⟨g⟩ is silent in phlegm but not in its derivatives like phlegmatic? After a research, I was linked to the Silent letter wiki:

Some are inert letters, which are sounded in a cognate word: e.g. ⟨n⟩ in damn (cf. damnation); ⟨g⟩ in phlegm (cf. phlegmatic); ⟨a⟩ in practically (cf. practical). If the cognate is obvious, it may aid writers in spelling, but mislead readers in pronunciation.

I can't find what is inert letters, so I would like to ask why do they appear? All I know is they are a part of a bigger set called dummy letters:

Dummy letters with no relation to neighbouring letters and no correspondence in pronunciation.

But searching for them only gives my a bunch of, erm, dummy links. There is no citation in the English part in the mentioned wiki so that I could research further.

Answer 1

I think the main point is that you may be asking the wrong question:

I would like to ask why do they appear?

They were there from the beginning. A better, and easier question to answer is:

Why did we stop pronouncing them?

Well, simply because we tend to simplify pronunciation in order to avoid breaking our tongues. Speakers of English generally have a hard time pronouncing some specific series of sounds, especially consonants.

The combination gm is an example of that, as is mn. If we can split the two consonants up, so they are pronounced in different syllables, there is no problem: it's not different from pronouncing the two sounds as if they are in different words:

He's a big man.
I don't like ham nor cheese.

However, when these two consonants end up in the same syllable, many speaker will have difficulty pronouncing them, so they will end up leaving out one of the letters.

Damn, but dam / nation
Plegm, but phleg / matic

As for the a in practically, that is simply a case where speakers tend to drop vowel sounds if they are not really necessary to aid pronunciation. Where gm is diffcult to pronounce for an English speaker, kl followed by a vowel sound is easy (think of clay, climate, etc.) However, kl at the end of a word is not easy, so there we do pronounce the a again.

What remains is of course the question

Why are those letters there in the first place, if they make words hard to pronounce?

Often that's because they were there in the version of the word as it existed in another language, from where English borrowed it. Often (some of) the original spelling of a word is retained when a word is borrowed, because it makes it easier for people to understand the origin (and hence, give them an idea about the meaning).

_{In some cases, the spelling of English words has even been changed to reflect this etymology, even when the pronunciation was wildly different. The word colonel is a great example of that. It was actually borrowed from Portuguese as coronel, and pronounced based on that spelling (ker'nel) but in order to reflect its Latin / Italian origin, the spelling was changed to colonel with an l. Nobody bothered to start pronouncing it that way though...}

Answer 2

English has many words borrowed and derived from other languages - French, German, Latin etc. The alphabets in English are also seen in other languages and so when words are borrowed from other languages (mostly as it is), we have to question the relevance of 'silent letters', not in English but in the 'native language'. Even some of the words like colour, honour etc have been less common to color or honor.

Reg 'phlegm', a google search on it says, 'Middle English fleem, fleume, from Old French fleume, from late Latin phlegma ‘clammy moisture (of the body)’, from Greek phlegma ‘inflammation’, from phlegein ‘to burn’. The spelling change in the 16th century was due to association with the Latin and Greek'.