A flag is a piece of fabric with a distinctive design that is usually rectangular and used as a symbol, as a signaling device, or decoration. The term flag is also used to refer to the graphic design employed by a flag, or to its depiction in another medium.
The first flags were used to assist military coordination on battlefields, and flags have since evolved into a general tool for rudimentary signalling and identification, especially in environments where communication is similarly challenging (such as the maritime environment where semaphore is used). National flags are potent patriotic symbols with varied wide-ranging interpretations, often including strong military associations due to their original and ongoing military uses. Flags are also used in messaging, advertising, or for other decorative purposes. The study of flags is known as vexillology, from the Latin vexillum meaning flag or banner.
In antiquity, field signs or standards were used in warfare that can be categorized as vexilloid or "flag-like". Examples include the Achaemenid battle standard Derafsh Kaviani, and the standards of the Roman legions such as the eagle of Augustus Caesar's Xth legion, or the dragon standard of the Sarmatians; the latter was let fly freely in the wind, carried by a horseman, but judging from depictions it was more similar to an elongated dragon kite than to a simple flag.
During the High Middle Ages flags came to be used primarily as a heraldic device in battle, allowing more easily to identify a knight than only from the heraldic device painted on the shield.Already during the high medieval period, and increasingly during the Late Middle Ages, city states and communes such as those of the Old Swiss Confederacy also began to use flags as field signs.Regimental flags for individual units became commonplace during the Early Modern period.
During the peak of the age of sail, beginning in the early 17th century, it has been customary (and later a legal requirement) for ships to carry flags designating their nationality; these flags eventually evolved into the national flags and maritime flags of today. Flags also became the preferred means of communications at sea, resulting in various systems of flag signals; see, International maritime signal flags.
Use of flags outside of military or naval context begins only with the rise of nationalist sentiment by the end of the 18th century; the earliest national flags date to that period, and during the 19th century it became common for every sovereign state to introduce a national flag.
See main article: National flag. One of the most popular uses of a flag is to symbolize a nation or country. Some national flags have been particularly inspirational to other nations, countries, or subnational entities in the design of their own flags. Some prominent examples include:
National flag designs are often used to signify nationality in other forms, such as flag patches.
See main article: Civil flag. A civil flag is a version of the national flag that is flown by civilians on non-government installations or craft. The use of civil flags was more common in the past, in order to denote buildings or ships that were not manned by the military. In some countries the civil flag is the same as the war flag or state flag, but without the coat of arms, such as in the case of Spain, and in others it's an alteration of the war flag.
See main article: War flag.
Other countries' armed forces (such as those of the United States or Switzerland) use their standard national flag. The Philippines' armed forces may use their standard national flag, but during times of war the flag is turned upside down. Bulgaria's flag is also turned upside down during times of war. These are also considered war flags, though the terminology only applies to the flag's military usage.
Four distinctive African flags currently in the collection of the National Maritime Museum in Britain were flown in action by Itsekiri ships under the control of Nana Olomu during conflict in the late 19th century. One is the flag generally known as the Benin flag and one is referred to as Nana Olomu's flag.
Please see Gallery of flags by design
See main article: Maritime flag. Flags are particularly important at sea, where they can mean the difference between life and death, and consequently where the rules and regulations for the flying of flags are strictly enforced. A national flag flown at sea is known as an ensign. A courteous, peaceable merchant ship or yacht customarily flies its ensign (in the usual ensign position), together with the flag of whatever nation it is currently visiting at the mast (known as a courtesy flag). To fly one's ensign alone in foreign waters, a foreign port or in the face of a foreign warship traditionally indicates a willingness to fight, with cannon, for the right to do so. As of 2009, this custom is still taken seriously by many naval and port authorities and is readily enforced in many parts of the world by boarding, confiscation and other civil penalties.
In some countries yacht ensigns are different from merchant ensigns in order to signal that the yacht is not carrying cargo that requires a customs declaration. Carrying commercial cargo on a boat with a yacht ensign is deemed to be smuggling in many jurisdictions.
There is a system of international maritime signal flags for numerals and letters of the alphabet. Each flag or pennant has a specific meaning when flown individually.
As well, semaphore flags can be used to communicate on an ad hoc basis from ship to ship over short distances.
Traditionally, a vessel flying under the courtesy flag of a specific nation, regardless of the vessel's country of registry, is considered to be operating under the law of her 'host' nation.
Flags are usually rectangular in shape (often in the ratio 2:3, 1:2, or 3:5), but may be of any shape or size that is practical for flying, including square, triangular, or swallow tailed. A more unusual flag shape is that of the flag of Nepal, which is in the shape of two stacked triangles. Other unusual flag shapes include the flag of Ohio and the flag of Tampa.
Many flags are dyed through and through to be inexpensive to manufacture, such that the reverse side is the mirror image of the obverse (front) side, generally the side displayed when the flag is flying from the observer's point of view from left, the side of the pole, to right. This presents two possibilities:
Some complex flag designs are not intended for through and through implementation, requiring separate obverse and reverse sides if made correctly. In these cases there is a design element (usually text) which is not symmetric and should be read in the same direction, regardless of whether the hoist is to the viewer's left or right. These cases can be divided into two types:
See also: Flags whose reverse differs from the obverse.
Common designs on flags include crosses, stripes, and divisions of the surface, or field, into bands or quarters—patterns and principles mainly derived from heraldry. A heraldic coat of arms may also be flown as a banner of arms, as is done on both the state flag of Maryland and the flag of Kiribati.
The de jure flag of Libya under Muammar Gaddafi, which consisted of a rectangular field of green, was for a long period the only national flag using a single color and no design or insignia. However, other historical states have also used flags without designs or insignia, such as the Soviet Republic of Hungary, whose flag was a plain field of red.
Colors are normally described with common names, such as "red", but may be further specified using colorimetry.
Vertical flags are sometimes used in lieu of the standard horizontal flag in central and eastern Europe, particularly in the German-speaking countries. This practice came about because the relatively brisk wind needed to display horizontal flags is not common in these countries.
The standard horizontal flag (no. 1 in the preceding illustration) is nonetheless the form most often used even in these countries.
The vertical flag (German: Hochformatflagge or Knatterflagge; no. 2) is a vertical form of the standard flag. The flag's design may remain unchanged (No. 2a) or it may change, e.g. by changing horizontal stripes to vertical ones (no. 2b). If the flag carries an emblem, it may remain centered or may be shifted slightly upwards. 
The vertical flag for hoisting from a beam (German: Auslegerflagge or Galgenflagge; no. 3) is additionally attached to a horizontal beam, ensuring that it is fully displayed even if there is no wind. 
The vertical flag for hoisting from a horizontal pole (German: Hängeflagge; no. 4) is hoisted from a horizontal pole, normally attached to a building. The topmost stripe on the horizontal version of the flag faces away from the building. 
The vertical flag for hoisting from a crossbar or banner (German: Bannerflagge; no. 5) is firmly attached to a horizontal crossbar from which it is hoisted, either by a vertical pole (no. 5a) or a horizontal one (no. 5b). The topmost stripe on the horizontal version of the flag normally faces to the left. 
See also: Religion in national symbols. Flags can play many different roles in religion. In Buddhism, prayer flags are used, usually in sets of five differently colored flags. Many national flags and other flags include religious symbols such as the cross, the crescent, or a reference to a patron saint. Flags are also adopted by religious groups and flags such as the Jain flag and the Christian flag are used to represent a whole religion.
As languages rarely have a flag designed to represent them, it is a common but unofficial practice to use national flags to identify them. Examples of this use include:
Though this can be done in an uncontroversial manner in some cases, this can easily lead to some problems for certain languages:
In this second case, common solutions include symbolising these languages by:
Thus, on the Internet, it is most common to see the English language associated to the flag of the United Kingdom, but sometimes to the flag of England, the flag of the United States or a US-UK mixed flag, usually divided diagonally.
Because of their ease of signalling and identification, flags are often used in sports.
See main article: Racing flags.
Social and political movements have adopted flags, to increase their visibility and as a unifying symbol.
The socialist movement uses red flags to represent their cause. The anarchist movement has a variety of different flags, but the primary flag associated with them is the black flag. In the 1970s, the rainbow flag was adopted as a symbol of the LGBT social movements. Bisexual and transgender pride flags were later designed, in an attempt to emulate the rainbow flag's success.
Some of these political flags have become national flags, such as the red flag of the Soviet Union and national socialist banners for Nazi Germany. The present Flag of Portugal is based on what had been the political flag of the Portuguese Republican Party previous to the 5 October 1910 revolution which brought this party to power.
Flags are often representative of an individual's affinity or allegiance to a country, team or business and can be presented in various ways. A popular trend that has surfaced revolves around the idea of the 'mobile' flag in which an individual displays their particular flag of choice on their vehicle. These items are commonly referred to as car flags and are usually manufactured from high strength polyester material and are attached to a vehicle via a polypropylene pole and clip window attachment.
In Australia, Canada, New Zealand, Philippines, and the United Kingdom a pair of red/yellow flags is used to mark the limits of the bathing area on a beach, usually guarded by surf lifesavers. If the beach is closed, the poles of the flags are crossed. The flags are colored with a red triangle and a yellow triangle making a rectangular flag, or a red rectangle over a yellow rectangle. On many Australian beaches there is a slight variation with beach condition signaling. A red flag signifies a closed beach (or, in the UK, some other danger), yellow signifies strong current or difficult swimming conditions, and green represents a beach safe for general swimming. In Ireland, a red and yellow flag indicates that it is safe to swim; a red flag that it is unsafe; and no flag indicates that there are no lifeguards on duty. Blue flags may also be used away from the yellow-red lifesaver area to designate a zone for surfboarding and other small, non-motorised watercraft.
Reasons for closing the beach include:
A surf flag exists, divided into four quadrants. The top left and bottom right quadrants are black, and the remaining area is white.
Signal flag "India" (a black circle on a yellow square) is frequently used to denote a "blackball" zone where surfboards cannot be used but other water activities are permitted.
Railways use a number of colored flags. When used as wayside signals they usually use the following meanings (exact meanings are set by the individual railroad company):
At night, the flags are replaced with lanterns showing the same colors.
Flags displayed on the front of a moving locomotive are an acceptable replacement for classification lights and usually have the following meanings (exact meanings are set by the individual railroad company):
A flagpole, flagstaff, or staff can be a simple support made of wood or metal. If it is taller than can be easily reached to raise the flag, a cord is used, looping around a pulley at the top of the pole with the ends tied at the bottom. The flag is fixed to one lower end of the cord, and is then raised by pulling on the other end. The cord is then tightened and tied to the pole at the bottom. The pole is usually topped by a flat plate or ball called a "truck" (originally meant to keep a wooden pole from splitting) or a finial in a more complex shape.Very high flagpoles may require more complex support structures than a simple pole, such as guy wires, or need be built as a mast.
Since 2011, the tallest free-standing flagpole in the world is the Dushanbe Flagpole in Tajikistan, with a height of 165 m (541 ft), beating the formerly record holding National Flagpole in Azerbaijan (size: 162 m; 532 ft) and the North Korean flag at Kijŏng-dong (size: 160 m; 525 ft).
The largest flag regularly hoisted in the world is the Brazilian national flag flown in the Square of the Three Powers in Brasilia, the Brazilian capital. This flag is about 3380 square meters (8740 square feet) and has never been taken down since opening in the mid 1960s.
Flagpoles can be designed in one piece with a taper (typically a steel taper or a Greek entasis taper), or be made from multiple pieces to make them able to expand. In the United States, ANSI/NAAMM guide specification FP-1001-97 covers the engineering design of metal flagpoles to ensure safety.
Hoisting the flag is to raise or haul up the flag with the help of a mechanical apparatus upon flag pole. Hoisting the flag either rising or lowering, especially national flag, usually involved ceremonies and certain set of rules depending on each countries. The flag raising squad is a group of people usually troops, cadets, or students that marching and bring the flags in flag hoisting ceremony. Flag hoisting ceremony involving flag raising squad can be simple or arranged elaborately involved large numbers of squad, usually during national or independence day ceremony.
Semaphore is a form of communication that utilizes flags. The signalling is performed by an individual using two flags (or lighted wands), the positions of the flags indicating a symbol. The person who holds the flags is known as the signalman. This form of communication is primarily used by naval signallers. This technique of signalling was adopted in the early 19th century and is still used in various forms today.
The colors of the flags can also be used to communicate. For example; a white flag means, among other things, surrender or peace, a red flag can be used as a warning signal, and a black flag can mean war, or determined to defeat enemies.
Orientation of a flag is also used for communication, though the practice is rarely used given modern communication systems. Raising a flag upside-down was indicative that the raising force controlled that particular area, but that it was in severe distress.
When blown by the wind, flags are subject to wave-like motions that grow in amplitude along the length of the flag. This is sometimes ascribed to the flag pole giving vortex shedding, however flags that are held by lanyards also can be seen to flap.