The flag of Uruguay is a bicolor of nine stripes with a golden sun in the canton, and both halves of the design point back to the same revolutionary decade in the Rio de la Plata.
Nineteen stripes, then nine
Uruguay's first flag came out of a law passed in December 1828, at the end of a long sequence that ran from Spanish colonial rule through a period of Brazilian control. That law set nine light blue stripes on a white field, one for each department the country then had, which alternate out to 19 bands in all. Nineteen bands on a flag is too many. The design was cluttered and hard to read at any distance.
A second law, on July 11, 1830, cut it to four blue stripes and five white, nine bands in all, and the nine departments are now counted in the bands rather than in the blue stripes alone. That is the flag Uruguay has flown since.
The Sun of May
Nine horizontal stripes alternate white and blue. In the upper hoist corner a white square carries the Sun of May in gold, with 16 rays that alternate between straight triangular points and wavy ones.
The sun comes from the May Revolution against Spanish rule and is associated with Inti, the Inca sun god. White is read as peace and honesty, blue as the sky and as liberty.
Uruguay's sun is the most obvious link to Argentina, which uses the same emblem; both countries trace their independence to the May Revolution. The striped field invites comparison with the flag of the United States, where stripes likewise stand for constituent parts of a country, though the two designs are unrelated in origin. Suns and similar motifs recur across the flags of former Spanish colonies in the region.
Where it flies
The flag is displayed on public buildings and schools and at national holidays and formal state occasions. Rules govern how it is raised, lowered and folded during ceremonies such as those on Independence Day.
There is also a presidential variant, which adds a laurel wreath around the Sun of May.
References
[1] National Library of Uruguay - Historical Archives
[2] Gobierno de Uruguay - Official Government Website
[3] Vexillological Association of the State of Uruguay