The flag of Greece (popularly referred to as the galanolefki or the kianolefki, the “blue-white”), officially recognized by Greece as one of its national symbols, is based on nine equal horizontal stripes of blue alternating with white.
There is a blue canton in the upper hoist-side corner bearing a white cross; the cross symbolizes Greek Orthodoxy, the established religion of the people of Greece and Greek Cypriots. According to popular tradition, the nine stripes represent the nine syllables of the phrase “Ελευθερία ή Θάνατος” (“Freedom or Death”, ” E-lef-the-ri-a i Tha-na-tos“), the five blue stripes for the syllables “Έλευθερία” and the four white stripes “ή Θάνατος”. The nine stripes are also said to represent the letters of the word “freedom” (Greek: Ελευθερία). There is also a different theory, that the nine stripes symbolize the nine Muses, the goddesses of art and civilization (nine has traditionally been one of the numbers of reference for the Greeks).
The blazon of the Greek flag is Azure, four bars Argent; on a canton of the field a Greek cross throughout of the second. The shade of blue used in the flag has varied throughout its history, from light blue to dark blue, the latter being increasingly used since the late 1960s.
The above patterns were officially adopted by the First National Assembly at Epidaurus on 13 January 1822. Blue and white have many interpretations, symbolizing the colours of the famed Greek sky and sea (combined with the white clouds and waves), traditional colours of Greek clothes in the islands and the mainland, etc. Wikipedia