The letter 'J' originated as a swash letter i, used for the letter 'i' at the end of Roman numerals when following another 'i', as in 'xxiij' instead of 'xxiii' for the Roman numeral representing 23. A distinctive usage emerged 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, 'I' and 'J' were different shapes for the same letter, both equally representing /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, acquired from the French J, has a sound value quite different from /j/ (which represents the initial sound in the English word "yet").
Use in English
In English, 'j' most commonly represents the affricate //. In Old English the phoneme /dʒ/ was represented orthographically as 'cg' or 'cȝ'. 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 loanwords 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 1633. In loan words such as raj, "J" may be pronounced /ʒ/ by some speakers. In some of these, including raj, Taj Mahal, and Beijing, the regular pronunciation /dʒ/ is actually closer to the foreign pronunciation, making this an instance of a hyperforeignism. Occasionally 'J' represents the original /j/ sound, as in Hallelujah and fjord (see: yodh for details). It also represents the /x/ (voiceless velar fricative) sound in Spanish loanwords such as ‘jalapeño’ (which is usually approximated by /h/ (voiceless glottal fricative) by native English speakers).
In marijuana, the ⟨j⟩ is silent. 
Use in other languages
The great majority of Germanic languages, such as German, Dutch, Icelandic, Swedish, Danish and Norwegian; use J for the palatal approximant /j/, which is usually represented by the letter y in English. Notable exceptions are English, Scots and (to a lesser degree) Luxembourgish. J also represents /j/ in Albanian, Baltic, and those Uralic and Slavic languages that use the Latin alphabet, such as Hungarian, Finnish, Estonian, Polish, Czech, Serbian, Slovak, Croatian, Latvian and Lithuanian. Some related languages, such as Serbian and Macedonian, also adopted J into the Cyrillic alphabet for the same purpose. Because of this standard, the minuscule letter was chosen to be used in the IPA as the phonetic symbol for the sound.
In the Romance languages J has generally developed from its original palatal approximant value in Latin to some kind of fricative. In French, Portuguese, Catalan, and Romanian it has been fronted to the postalveolar fricative /ʒ/ (like s in English measure). In Spanish, by contrast, it has been both devoiced and backed from an earlier /ʝ/ to a present-day /x ~ h/, with the actual phonetic realization depending on the speaker's dialect/s.
In modern standard Italian spelling, only Latin words, proper nouns (such as Jesi, Letojanni, Juventus etc.) or those of foreign languages have J. Until the 19th century, J was used instead of I in diphthongs, as a replacement for final -ii, and in vowel groups (as in Savoja); this rule was quite strict for official writing. J is also used to render /j/ in dialect, e.g. Romanesque ajo for standard aglio (–/ʎ/–) (garlic). The Italian novelist Luigi Pirandello used J in vowel groups in his works written in Italian; he also wrote in his native Sicilian language, which still retains the J to represent /j/ (and sometimes also [dʒ] or [gj], depending on its environment).
Among non-European languages which have adopted the Latin script, 'J' stands for /ʒ/ in Turkish, Azerbaijani, and Tatar. 'J' stands for /dʒ/ in Indonesian, Somali, Malay, Igbo, Shona, Oromo, Turkmen, and Zulu. It represents a voiced palatal plosive /ɟ/ in Konkani, Yoruba, and Swahili. In Kiowa, 'J' stands for a voiceless alveolar plosive, /t/.
In Chinese Pinyin, 'J' stands for /tɕ/, an unaspirated Q. The Royal Thai General System of Transcription does not use the 'J', although it is used in some proper names and non-standard transcriptions to represent either จ [tɕ] or ช [tɕʰ] (the latter following Pali/Sanskrit root equivalents). In romanized Pashto, 'J' represents ځ, pronounced [dz].
The capital letter J, in upright (non-slanted) type, is the unit symbol for the joule energy unit. It is also the dimension symbol for luminous intensity, conventionally rendered in a sans-serif typeface.
|Unicode name||LATIN CAPITAL LETTER J||LATIN SMALL LETTER J||LATIN SMALL LETTER DOTLESS J|
|UTF-8||74||4A||106||6A||200 183||C8 B7|
|Numeric character reference||J||J||j||j||ȷ||ȷ|
- 1 Also for encodings based on ASCII, including the DOS, Windows, ISO-8859 and Macintosh families of encodings.
Unicode also has a dotless variant, ȷ (U+0237) for use with combining diacritics.
In Unicode, a duplicate of 'J' for use as a special phonetic character in historical Greek linguistics is encoded in the Greek script block as ϳ (Unicode U+03F3). It is used to denote the palatal glide /j/ in the context of Greek script. It is called "Yot" in the Unicode standard, after the German name of the letter J. An uppercase version of this letter was added to the Unicode Standard at U+037F with the release of version 7.0 in June 2014.
Wingdings smiley issue
In the Wingdings font by Microsoft, the letter "J" is rendered as a smiley face (note this is distinct from the Unicode code point U+263A, which renders as ☺). In Microsoft applications, an engineer corrected ":)" as a smiley rendered in a specific font face when composing rich text documents (and/or HTML email). This autocorrection feature can be switched off or changed to a Unicode smiley. 
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- "J" and "jay", Merriam-Webster's Third New International Dictionary of the English Language, Unabridged (1993)
- Mittelhochdeutsches Handwörterbuch von Matthias Lexer (1878)
- De le lettere nuωvamente aggiunte ne la lingua Italiana in Italian Wikisource.
- Hogg, Richard M.; Norman Francis Blake; Roger Lass; Suzanne Romaine; R. W. Burchfield; John Algeo (1992). The Cambridge History of the English Language. Cambridge University Press. p. 39. ISBN 0-521-26476-6.
- English Grammar, Charles Butler, 1633
- Wells, John (1982). Accents of English 1: An Introduction. Cambridge, UN: Cambridge University Press. p. 108. ISBN 0-521-29719-2.
- "marijuana". Cambridge Dictionaries Online. Cambridge University Press. Retrieved 23 April 2015.
- Penny, Ralph John (2002). A History of the Spanish Language. Cambridge, UK: Cambridge University Press. ISBN 0-521-01184-1.
- Cipolla, Gaetano (2007). The Sounds of Sicilian: A Pronunciation Guide. Mineola, NY: Legas. pp. 11–12. Retrieved 2013-03-31.
- Nick Nicholas, "Yot"
- Unicode code chart for Greek
- "Unicode: Greek and Coptic" (PDF). Retrieved 2014-06-26.
- "Unicode 7.0.0". Unicode Consortium. Retrieved 2014-06-26.
- Raymond Chen (23 May 2006). "That mysterious J". The Old New Thing. MSDN Blogs. Retrieved 2011-04-01.
|Wikisource has the text of the 1911 Encyclopædia Britannica article J.|
- Media related to J at Wikimedia Commons
- The dictionary definition of J at Wiktionary
- The dictionary definition of j at Wiktionary