According to some authorities, it is the sixth (or seventh if including W) vowel letter of the English alphabet.

In Latin, Y was named I graeca ("Greek I"), since the classical Greek sound /y/, similar to modern German ü or French u, was not a native sound for Latin speakers, and the letter was initially only used to spell foreign words.

This history has led to the standard modern names of the letter in Romance languages – i grego in Galician, i grega in Catalan, i grec in French and Romanian, and i greca in Italian – all meaning "Greek I".

[4] The original Greek name, υ ψιλόν (upsilon), has also been adapted into several modern languages.

France has a commune called Y, pronounced /i/ ⓘ, whose inhabitants go by the demonym upsilonienne/upsilonien in feminine and masculine form respectively.

It was used to transcribe loanwords from the Attic dialect of Greek, which had the non-Latin vowel sound /y/ (as found in modern French cru (raw) or German grün (green)) in words that had been pronounced with /u/ in earlier Greek.

It occurs mainly in the following three environments: for upsilon in Greek loan-words (system: Greek σύστημα), at the end of a word (rye, city; compare cities, where S is final), and in place of I before the ending -ing (dy-ing, ty-ing).

In the West Slavic languages, ⟨y⟩ was adopted as a sign for the close central unrounded vowel /ɨ/; later, /ɨ/ merged with /i/ in Czech and Slovak, whereas Polish retains it with the pronunciation [ɘ].

In Alemannic German names, it denotes long /iː/, for instance in Schnyder [ˈʃniːdər] or Schwyz [ˈʃʋiːts] – the cognate non-Alemannic German names Schneider [ˈʃnaɪdər] or Schweiz [ʃʋaɪts] have the diphthong /aɪ/ that developed from long /iː/.

In Hungarian orthography, y is only used in the digraphs "gy", "ly", "ny", "ty", in some surnames (e.g. Bátory), and in foreign words.

In Faroese, too, the contrast has been lost, and ⟨y⟩ is always pronounced /i/, whereas the accented versions ⟨ý⟩ and ⟨í⟩ designate the same diphthong /ʊi/ (shortened to /u/ in some environments).

In French orthography, ⟨y⟩ is pronounced as [i] when a vowel (as in the words cycle, y) and as [j] as a consonant (as in yeux, voyez).

The letter ⟨y⟩ has double function (modifying the vowel as well as being pronounced as [j] or [i]) in the words payer, balayer, moyen, essuyer, pays, etc., but in some words it has only a single function: [j] in bayer, mayonnaise, coyote; modifying the vowel at the end of proper names like Chardonnay and Fourcroy.

Hence, el yugo y las flechas was a symbol sharing the initials of Isabella I of Castille (Ysabel) and Ferdinand II of Aragon.

It was always common for Brazilians to stylize Tupi-influenced names of their children with the letter (which is present in most Romanizations of Old Tupi) e.g. Guaracy, Jandyra, Mayara – though placenames and loanwords derived from indigenous origins had the letter substituted for ⟨i⟩ over time e.g. Nictheroy became Niterói.

Usual pronunciations are /i/, [j], [ɪ] and /ɨ/ (the two latter ones are inexistent in European and Brazilian Portuguese varieties respectively, being both substituted by /i/ in other dialects).

The letter is also common in some surnames native to the German-speaking province of Bolzano, such as Mayer or Mayr.

It can also represent Tudor and Revived Late Cornish [ɛ] and [eː] and consequently be replaced in writing with ⟨e⟩.

In Aymara, Indonesian/Malaysian, Turkish, Quechua and the romanization of Japanese, ⟨y⟩ is always a palatal consonant, denoting [j], as in English.