Old French phonology

Old French was constantly changing and evolving. However, the form in the late 12th century, as attested in a great deal of mostly poetic writings, can be considered standard. The writing system at this time was more phonetic than that used in most subsequent centuries. In particular, all written consonants (including final ones) were pronounced, except for s preceding non-stop consonants and t in et, and final e was pronounced. The phonological system can be summarised as follows:

Consonants
Notes:
 * The affricates, , , became fricatives  in Middle French.  was written as c, ç, -z, as in cent, chançon, priz ("a hundred, song, price"). /dz/ was written as -z-, as in doze "twelve".
 * (l mouillé), as in conseil, travaillier ("advice, to work"), became in Modern French.
 * appeared not only in the middle of a word, but also at the end, as in poing "hand". At the end of a word, was later lost, leaving a nasalized vowel.
 * was found only in Germanic loanwords and was later lost. In native Latin words, was lost early on, as in om, uem, from Latin homō.
 * Intervocalic from both Latin  and  was lenited to  in the early period (cf. contemporary Spanish: amado ). At the end of words it was also devoiced to . In some texts it was sometimes written as dh or th (aiudha, cadhuna, Ludher, vithe). By 1100 it disappeared altogether.

Vowels
In Old French, the nasal vowels were not separate phonemes, but occurred as allophones of the oral vowels before a nasal stop. This nasal stop was fully pronounced; thus bon was pronounced (Modern French ). Nasal vowels were present even in open syllables before nasals, where Modern French has oral vowels, as in bone (Modern French bonne ).

Monophthongs
Notes:
 * had formerly existed, but closed to ; it would later appear again when monophthongized, and also when  closed in certain positions (e.g. when followed by original  or, but not by , which later became ).

Diphthongs and triphthongs
Notes:
 * In Early Old French (up to about the mid-12th century), the spelling $\langleai\rangle$ represented a diphthong, instead of the later monophthong , and $\langleei\rangle$ represented the diphthong , which became in Late Old French.
 * In Early Old French the diphthongs described above as "rising" may have been falling diphthongs . In earlier works with vowel assonance, the diphthong written $\langleie\rangle$ did not assonate with any pure vowels, suggesting that it cannot have simply been.
 * The pronunciation of the vowels written $\langleue\rangle$ and $\langleeu\rangle$ is debated. In very early Old French, they represented (and were written as), and by Middle French, they had both merged as , but it is unclear what the transitional pronunciations were.