The large triangle on the hoist side of the Czech Republic’s national flag is blue. It extends halfway along the flag and separates the white upper field from the red lower field.
The flag’s white and red colours originated with the historic Kingdom of Bohemia. After Czechoslovakia was established in 1918, a blue triangle was added in 1920 to prevent confusion with Poland’s white-and-red flag and to represent Slovakia.
The Czech Republic retained the Czechoslovak flag after the peaceful dissolution of Czechoslovakia in 1993. The design is often described as a white-over-red bicolour with a blue hoist triangle. Its proportions and the triangle’s exact geometry are defined by law, making the blue element an intentional structural feature rather than a small emblem.