The first flag of the Ukrainian Soviet Socialist Republic was adopted on March 10, 1919, to serve as the symbol of state of the Ukrainian SSR. Details of the official flag changed periodically before the break-up of the Soviet Union in 1991, but all were based on the red flag of the Bolshevik Revolution.
The first flag was red with the gold Cyrillic sans-serif letters У.С.С.Р. (USSR, acronym for Ukrayinskaya Sotsialisticheskaya Sovetskaya Respublika in the Russian language). In the 1930s a gold border was added. In 1937, a new flag was adopted, with a small gold hammer and sickle added above the gold Cyrillic serif letters У.Р.С.Р. (URSR, for Ukrayins’ka Radyans’ka Sotsialistychna Respublika in the Ukrainian language).
The Soviet Union and two of its republics (the Ukrainian SSR and the Byelorussian SSR) all became members of the nascent United Nations (UN) in 1945. Since all of their flags were red with only small markings in upper left corner, the UN demanded changes to the flags in 1949. To comply, the Ukrainian Soviet authorities dropped the lettering and added a blue stripe (⅓ of the width). The Ukrainian SSR adopted this new design as its official flag on November 21, 1949. Other constituent republics of the Soviet Union soon followed suit and customized the bottom third of their flags.