The case of "Donc"

Donc

If we abide by the "Lucifer" rule on final pronounced consonants in the French language, the word "donc" should see the final sound [k] pronounced every time.

However, there is a little more to this word. In old French, it was pronounced and written "doncques" [dõkœ]. Over the years, through poetry and diverse writings, the use of what we call "licence poétique" (in English, poetic freedom) effectively and permanently modified the way we spell the word to "donc".
"Licence poétique" is a poet's choice of changing the pronunciation and sometimes the spelling of a word, to keep the consistency of the poetic composition (prosody and versification). An example you might sometimes see is the word "encore", written as "encor".

The way I approach "donc" is as follows: if it starts a sentence and serves the purpose of introducing a conclusion, like in Carmen's Séguedille "Donc, pour me tenir compagnie" = "So, to keep me company, etc" -> I always ask for it to be pronounced.
When it is integral part of a phrase, I look at the character. If the character is educated, of higher social rank and/or older, not pronouncing might support its status. On the other hand, if the character is younger, from a more plebeian social class and/or little to not educated, then having it pronounced is more realistic as well.

Basically it comes down to deciding whether the character would use "licence poétique" or if it is appropriate at that time, which can be highly subjective.

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