Designing a flag means solving two problems at once. The flag has to be recognizable at a distance, in motion, and printed on whatever material is to hand. It also has to mean something to the people it represents, whether that is a nation, a city, an organization or a cause. Most of the established design principles exist because those two demands pull in opposite directions.
Simplicity
Simplicity comes first. The most effective flags are the ones anyone can recognize and redraw, children included. That is what keeps a flag distinct when it is seen from far away, moving in wind, or reproduced badly.
The Swiss flag is the case in point: a white cross centered on a square red field. There is almost nothing to it, and that is why it works. It survives every reproduction it is put through and remains unmistakably Swiss.
Symbolism
Every element in a flag, from color to shape, is chosen for what it means. Color does most of the work. Red often denotes valor and bravery, blue vigilance and justice, white purity and peace. Motifs carry their own conventions: stars for unity or excellence, crescents for a connection to Islam, triangles for strength and resilience.
South Africa's flag shows how much a design can hold. A Y-shaped green stripe edged in white sits on a field of black, gold, green, white, red and blue. The Y stands for diverse elements in South African society converging and moving forward together, and the colors between them cover the country's identity, its history and its natural wealth.
Space and proportion
Proportion affects how a flag is read. The width-to-length ratio changes how it looks on a pole and how it is displayed, and the layout of colors and symbols inside that space determines whether the design stays legible.
Canada's flag is a good example of the arithmetic working out. Two vertical red bands flank a white square with a red maple leaf inside it. The proportions and placement are balanced closely enough that the flag reads instantly at any size.
Adaptability
A flag has to survive political and social change without becoming an embarrassment. That argues for symbols with some staying power and against anything so specific it dates. There is a physical version of the same requirement: the design has to stay clear in bright sun and bad weather, which is partly a question of durable, fade-resistant materials.
Emotional resonance
The last thing a flag needs is the hardest to design for. It has to produce a response, some mix of pride, solidarity and commitment, in the people it stands for. That is the difference between a symbol and an icon that can move people. The Stars and Stripes does this for Americans, carrying patriotism and shared national identity along with the history of the country, none of which is written anywhere on the cloth.