From
here:
J originated as a swash character to end some Roman numerals in place of i. There was an emerging distinctive use in Middle High German. Gian Giorgio Trissino (1478-1550) was the first to explicitly distinguish I and J as representing separate sounds, in his
Ɛpistola del Trissino de le lettere nuωvamente aggiunte ne la lingua italiana (“Trissino’s epistle about the letters recently added in the Italian language”) of 1524. Originally, both I and J represented /i/, /iː/, and /j/; but Romance languages developed new sounds (from former /j/ and /ɡ/) that came to be represented as I and J; therefore, English J (from French J) has a sound value quite different from /j/ (which represents the sound in the English word “
yet”).
In English J most commonly represents the affricate /dʒ/ (as in jet). In Old English the phoneme /dʒ/ was represented orthographically as cʒ. Under the influence of Old French, which had a similar phoneme deriving from Latin /j/, English scribes began to use I (later J) to represent word-initial /dʒ/ of Old English (for example,
iest, later
jest), while using DG elsewhere (for example,
hedge). Later many other uses of I (later J) were added in loan words from French and other languages (e.g.
adjoin,
junta). The first English-language book to make a clear distinction between I and J was published in 1634. In loanwords such as
jam, “J” may be pronounced /ʒ/ by some, but not all, speakers. In some such cases, including
raj,
Taj Mahal and others, the regular /dʒ/ is actually closer to the original sound of the foreign language, making this realization a hyperforeignism. Occasionally
J represents other sounds, as in Hallelujah which is pronounced the same as “Halleluyah” (See the Hebrew yud for more details).