A white cross on a red square. The Swiss flag is one of the few square national flags in the world, and its plainness is a large part of why it is recognized everywhere.
A mark shared by the cantons
The white cross first appears in the 14th century as a sign of a pact between cantons at war, a way of marking allies drawn from territories that had little else in common. It attached itself to the Swiss Confederation at the Battle of Laupen in 1339. For centuries afterwards the design shifted along with the cantonal flags it sat beside.
The current flag dates to 1889, four decades after the Federal Constitution of 1848 turned a loose confederation into a federal state. A single agreed flag gave the cantons something to hold in common and set Switzerland apart from its neighbors.
The square and the cross
The cross sits equidistant from the edges of a red square. The white cross is a Christian emblem and a sign of Swiss independence. Red stands for freedom and democracy, and for the blood of the Swiss who died fighting for the country's autonomy in earlier centuries. Both readings pull toward the neutrality and peace that have defined Swiss policy since.
Where it flies
Government offices, the army and national celebrations all use the flag, and it hangs from public buildings year round. At international events, including the Olympic Games, its square shape makes it easy to pick out of a row of national flags. Civil and state versions exist to distinguish private enterprises from state institutions. Protocol asks that the flag be treated with respect, and commercial advertising needs authorization.
The Red Cross and other borrowings
The Red Cross emblem was drawn directly from Swiss symbolism, and that borrowing has done a great deal for Switzerland's association with humanitarian work. The Nordic flags also use crosses, though with different proportions and off-center arms, and cross motifs were common across the neighboring regions that Switzerland grew up among. In marketing and design, the flag has become shorthand for quality and precision, and Swiss artists have used it to work through the country's ideas about neutrality.
References
[1] Swiss Federal Chancellery official publications. (www.admin.ch)
[2] International Federation of Vexillological Associations. (www.fiav.org)
[3] Swiss National Museum historical records. (www.nationalmuseum.ch)