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Greek alphabet
Αα Alpha	Νν Nu
Ββ Beta	Ξξ Xi
Γγ Gamma	Οο Omicron
Δδ Delta	Ππ Pi
Εε Epsilon	Ρρ Rho
Ζζ Zeta	Σσ Sigma
Ηη Eta	Ττ Tau
Θθ Theta	Υυ Upsilon
Ιι Iota	Φφ Phi
Κκ Kappa	Χχ Chi
Λλ Lambda	Ψψ Psi
Μμ Mu	Ωω Omega
Obsolete letters
Ϝϝ Digamma	Ϸϸ Sho
Ϛϛ Stigma	Ϙϙ Qoppa
Ϻϻ San	Ϡϡ Sampi
Greek diacritics

This article is about Greek upsilon. For the Latin upsilon, see ʊ.
Upsilon (uppercase Υ, lowercase υ) is the 20th letter of the Greek alphabet. In the system of Greek numerals it has a value of 400. It is derived from the Phoenician waw.

In early Greek it was pronounced like English oo, IPA: [u] . In Classical Greek, it was pronounced like French u or German ü, IPA: [y] — a sound that is not found in most dialects of English. In Modern Greek it is pronounced like continental i or English ee, IPA: [i], and in diphthongs, [f] or [v]. In ancient Greek it occurred in both long and short versions, but this distinction has been lost in Modern Greek.

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 which 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: for instance after alpha or epsilon it is pronounced [f] or [v].

The Roman Emperor Claudius proposed introducing a new letter into the Latin alphabet to approximate the sound of upsilon, but in due course the letter Y was adopted instead.

The name of the letter was originally just υ. It changed to u psilon (Greek υ ψιλόν, meaning 'simple u') to distinguish it from ου, which had come to have the same [y] pronunciation. The pronunciation of the name of the letter in modern Greek sounds in English like "EEP-silon" (in contrast to the letter Ε, which is pronounced "EPP-silon"). It is also rarely called "ypsilon" (IPP-silon) in English because of its resemblance to the Roman letter Y.

Four letters of the Latin alphabet arose from it: V and Y and, much later, U and W. In the Cyrillic alphabet, the letters U (у) and Izhitsa (ѵ) arose from it.

• In particle physics the capital Greek letter Υ denotes a meson. Note that the symbol should always look like ϒ in order to avoid confusion with a Latin Y denoting the hypercharge.
• Automobile manufacturer Lancia has a model called the Ypsilon. See Lancia Ypsilon.