Mute e (French prosody)
The final "e" that counts or vanishes depending on position: the central rule — and main trap — of French syllable counting.
The mute e (e caduc) is the final "e" modern French barely pronounces, but which classical metrics keeps alive under three rules: before a consonant it counts (the e of "rose" in "une rose rouge" is one syllable); before a vowel it elides and doesn't count; at the line's end it never counts.
It's French verse's great specificity — and the first source of counting errors. It's also its intimate music: the counted e lengthens and softens, giving the line the suppleness traditional poetic diction lets you hear. Verlaine drew unmatched blur effects from it. In song and slam, its treatment remains a fully expressive choice: sounding or swallowing the e's is choosing a scansion.
Example
Compare English "-ed" endings in older verse: "belovèd" sounded, "beloved" not.