The name of the letter was originally just "υ" (y; also called hy, hence "hyoid", meaning "shaped like the letter υ"), but the name changed to "υ ψιλόν" u psilon 'plain υ' to distinguish it from οι, which had come to have the same [y] pronunciation.[7]
As an initial letter in Classical Greek, it always carried the rough breathing (equivalent to h) as reflected in the many Greek-derived English words, such as those that begin with hyper- and hypo-. This rough breathing was derived from an older pronunciation that used a sibilant instead; this sibilant was not lost in Latin, giving rise to such cognates as super- (for hyper-) and sub- (for hypo-).
Upsilon participated as the second element in falling diphthongs, which have subsequently developed in various ways.
The usage of Y in Latin dates back to the first century BC. It was used to transcribe loanwords from Greek, so it was not a native sound of Latin and was usually pronounced /u/ or /i/. The latter pronunciation was the most common in the Classical period and was used mostly by uneducated people. The Roman EmperorClaudius proposed introducing a new letter into the Latin alphabet to transcribe the so-called sonus medius (a short vowel before labial consonants), but in inscriptions, the new letter was sometimes used for Greek upsilon instead.
In some languages, including German and Portuguese, the name upsilon (Ypsilon in German, ípsilon in Portuguese) is used to refer to the Latin letter Y as well as the Greek letter. In some other languages, the (Latin) Y is referred to as a "Greek I" (i griega in Spanish, i grec in French), also noting its Greek origin.
In particle physics the capital Greek letter ϒ denotes an Upsilon particle. Note that the symbol should always look like in order to avoid confusion with a Latin Y denoting the hypercharge. This may be done either with a font such as FreeSerif or with the dedicated Unicode character U+03D2 ϒ.
Automobile manufacturer Lancia has a model called the Ypsilon.
Upsilon is known as Pythagoras' letter, or the Samian letter, because Pythagoras used it as an emblem of the path of virtue or vice.[13] As the Roman writer Persius wrote in Satire III:
and the letter which spreads out into Pythagorean branches has pointed out to you the steep path which rises on the right.[14]
Lactantius, an early Christian author (ca. 240 – ca. 320), refers to this:
For they say that the course of human life resembles the letter Y, because every one of men, when he has reached the threshold of early youth, and has arrived at the place "where the way divides itself into two parts," is in doubt, and hesitates, and does not know to which side he should rather turn himself.[15]
^F. Lauritzen, "Michael the Grammarian's irony about Hypsilon. A step towards reconstructing Byzantine pronunciation", Byzantinoslavica, 67 (2009)
^Mihalas and McRae (1968), Galactic Astronomy (W. H. Freeman)
^Walpole, Ronald (2017). Probability and Statistics for Scientists and Engineers (9th ed.).
^Brewer, Ebenezer Cobham. The reader's handbook of famous names in fiction, allusions, references, proverbs, plots, stories, and poems, Vol. 2, p. 956. Lippincott, 1899.