Monaco’s flag has the same red-over-white design as Indonesia’s flag, but the two countries use different proportions. Monaco’s flag is more compact, with a 4:5 ratio, while Indonesia’s is 2:3.
The resemblance is largely coincidental. Monaco’s colours come from the heraldic red and white of the House of Grimaldi, recorded since at least 1339. The present plain bicolour was adopted in 1881 under Prince Charles III. Indonesia’s red-and-white symbolism is linked to older Javanese and Majapahit traditions and was used prominently during the country’s independence movement.
At a distance, the flags can be difficult to distinguish, especially because both place red above white and have no central emblem. Their proportions provide the clearest test: Monaco’s flag is relatively taller and squarer, whereas Indonesia’s is longer. Poland is another frequent mix-up, but its colours are reversed, with white above red.