High Quality Content by WIKIPEDIA articles Upsilon (uppercase Υ, lowercase υ; Greek: Ύψιλον, psilon) 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. The name of the letter is pronounced in Modern Greek, and in English /ˈʌpsɨlɒn/, UK: /juːpˈsaɪlən/ yewp-SY-lən, or US: /ˈjuːpsɨlɒn/...