The flag of Luxembourg (Luxembourgish: Lëtzebuerger Fändel, German: Flagge Luxemburgs, French: Drapeau du Luxembourg) consists of three horizontal stripes, red, white and blue, and can be in 1:2 or 3:5 ratio.
The flag of Luxembourg was first used between 1845 and 1848 and officially adopted on June 23, 1972. Luxembourg had no flag until 1830, when patriots were urged to display the national colors. The flag was defined as a horizontal tricolor of red, white, and blue in 1848, but it was not officially adopted until 1972. The tricolor flag is almost identical to that of the Netherlands, except that it is longer and its blue stripe is a lighter shade. The red, white, and blue coloring was derived from the Grand Duke’s coat of arms, which dates from the 13th century.
The colours of the flag of Luxembourg were first adopted around 1830 during the Belgian Revolution. They were probably derived from the counts, dukes and later grand-dukes of Luxembourg’s coat of arms which in turn was derived from the combination of the dukes of Limbourg’s Lion and the supposed striped banner of the early counts of Luxembourg. The three-colored horizontal design was fixed on June 12, 1845.
It took until June 23, 1972 before a law was passed regulating the flag of Luxembourg. The same law also prescribed ensign and roundel for aircraft and ships registered in Luxembourg. Wikipedia