Portugal's flag is one of the harder national flags to draw from memory. Adopted on June 30, 1911, it puts a medieval shield, seven gold castles, and a brass navigational instrument over a green and red field that had been a party color a year earlier.
From Braganza blue to republican green
The revolution of 1910 removed the monarchy, and the blue and white of the House of Braganza went with it. The commission that designed the replacement included João Chagas and Abel Botelho, and it settled on green and red, the colors of the Republican movement. The new flag was adopted on June 30, 1911.
Earlier Portuguese flags had carried the royal arms or a single shield on a plain field. The 1911 design kept the shield and moved it onto colors with no monarchical history behind them, which was the point.
Reading the coat of arms
The field splits vertically, green over the hoist two-fifths and red over the remaining three-fifths, with the coat of arms straddling the line between them.
At the center is the Portuguese shield: five small blue shields arranged in a cross, each marked with five white dots, inside a red border carrying seven gold castles. The five blue shields go back to the legend of Afonso I's victory over the Moors.
Behind the shield sits the armillary sphere, the navigational instrument that ties the flag to the Age of Discovery and the voyages of Henry the Navigator's era and after. Green on the flag is read as hope, red as the blood of those who died for the country.
Flying it
The flag flies at government buildings, embassies, and diplomatic events, and on public holidays. Portuguese law sets the protocol: when it goes up and comes down, where it is placed, how it hangs. Military ensigns adapt the design with branch insignia while keeping the core elements. Away from official use it appears on balconies, at football matches, and through Portuguese art and film.
The sphere elsewhere
The armillary sphere reappears in the flags and emblems of Lusophone countries, which kept parts of the same symbolic vocabulary after independence. Within Europe the Portuguese flag has no close relatives. Most of the continent settled on plain bars and crosses.
References
"Flags of the World" by Whitney Smith, Simon & Schuster
Vexillological Records from FIAV
"Portugal: A Companion History" by José Hermano Saraiva