Both public and private buildings such as schools and courthouses often fly the national flag. In some countries, the national flags are only flown from non-military buildings on certain flag days.
There are three distinct types of national flag for use on land, and three for use at sea, although many countries use identical designs for several (and sometimes all) of these types of flag.
On land, there is a distinction between civil flags (FIAV symbol ), state flags and war or military flags (). State flags are those used officially by government agencies, whereas civil flags may be flown by anyone irrespective of whether they are linked to government. War flags (also called military flags) are used by military organisations such as armies.
In practice, many countries (including the United States and the United Kingdom) have identical flags for these three purposes; national flag is sometimes used as a vexillological term to refer to such a three-purpose flag (). In a number of countries, however—notably those in Latin America—there is a distinct difference between civil and state flags. In most cases, the civil flag is a simplified version of the state flag, the difference often being the presence of a coat of arms on the state flag which is absent from the civil flag.
Very few countries use a war flag that differs from the state flag; the PRC, ROC, and Japan are notable exceptions. The Philippines does not have a distinctive war flag in this usual sense, but the flag of the Philippines is unique in that it is flown with the red stripe on top, rather than the blue, when the country is in a state of war.
In some countries, such as the United States and Canada, the national ensign is identical to the national flag, while in others, such as the United Kingdom and Japan, there are specific ensigns for maritime use. Most countries do not have a separate state ensign, although the United Kingdom is a rare exception, in having a red ensign for civil use, a white ensign as its naval ensign, and a blue ensign for government non-military vessels.
There are three color combinations that are used on several flags in certain regions. Red, white and blue are common amongst Western nations, including France, the United Kingdom, Australia and the United States of America. Many African nations use red, yellow and green, including Senegal, Mali and Cameroon. Flags containing red, white and black can be found particularly amongst Arab nations such as Iraq, Yemen and Egypt.
While some similarities are coincidental, others are rooted in shared histories. For example, the flags of Venezuela, of Colombia, and of Ecuador all use variants of the flag of Great Colombia, the country they composed upon their independence from Spain, created by the Venezuelan independence hero Francisco de Miranda; and the flags of Egypt, of Iraq, of Syria, and of Yemen are all highly similar variants of the flag of the Arab revolt of 1916–1918. The Nordic countries all have the same design (Iceland, Denmark, Norway, Sweden, Finland, in addition to the autuonomus regions of the Faroe Islands and Åland), a cross on a single-colored background. The USA and UK both have red, white, and blue. This similarity is due to the fact that the first 13 states of the USA were former colonies of the United Kingdom. Also, Australia and New Zealand both share a very similar flag, they are both based on their British heritage. Both flags feature the Union Flag, both have royal blue background and both have the Southern Cross as a prominent feature. The only two differences between these flags is that the Australian flag have the Commonwealth Star, and that on the New Zealand flag, the Southern Cross is red instead of white.
Many other similarities may be found among current national flags, particularly if inversions of color schemes are considered (e.g., compare the flag of Côte d'Ivoire to that of Ireland). Still more identical or closely similar pairs exist comparing present day and historical flags; for example, the current national flag of Albania was the war flag of the Byzantine (Eastern Roman) Empire.
The flag of Nepal is the only national flag which is non-rectangular.
The flag of Libya is the only national flag which consists of just one solid color (green) with no other designs or symbols.
The flags of Cyprus and Kosovo are the only national flags which depict the shape of the country that it represents (The Korean Unification Flag depicts the Korean Peninsula although it is not an official flag in either North or South Korea).
The flag of the Philippines is the only flag which may be hoisted upside-down when its Congress has declared a "state of war" (It is usually flown with the blue stripe over the red; when at war, it is the red over the blue).