WHY DO MANY FLAGS LOOK SO SIMILAR?

Something useful to know.

In most cases that flags are similar you see that these countries are very closely related either historically, culturally or ethnically. Usually they are descended from the same people group. Other times it because of shared definition of common symbols or colors. 

Let's first look at some of the flags of Europe and see patterns that emerge. I would like to start off with the world famous Union Jack which ties together elements of many nations into a single flag and which has become the predecessor to flags across the world. By understanding the Union Jack you can start to understand how the symbols for national identity are related throughout the world and why flags all over the world look so similar.

This is the modern flag of the United Kingdom and it is actually a rather interesting study in the evolution of flags in Europe. We won't begin with the Union Jack though, but start somewhere much older that puts the roots to many European standards in a more ancient place.

I would like to start in the tiny country of Denmark:

The Scandinavian countries all show a similar use of crosses and for the most part color in their selection of flags, probably owed to the expansion and the establishment of the Kingdom of Denmark.

Denmark:

Norway:

Finland:

Sweden

Following the early period of Danish and Scandinavian advance you see expansion from the Scandinavian countries into other parts of Europe. Through later conquest and colonization these nations influenced flags like that of Iceland and the countries that one day form the United Kingdom.

Iceland: (It's the exact opposite of Norway)

It also gave the British Isles many of their flags, which still exist in one form today. The modern UK flag is comprised of a marrying of at least three different historical flags (St Andrew's Cross, St George's Cross and St Patrick's Cross) 

Kingdom of Scotland (St Andrew's Cross)

England (St George's Cross)

These two led to the creation of flag of Great Britain. This is the original "Union Jack" named as the flag that signified King James I joining of the Kingdoms of England and Scotland, which he was the ruler of both by the twists and turns of European monastic destiny. James was the first legitimate heir to both the crowns of Scotland and England. With the death of Queen Victoria I, King James of Scotland ascended to the rank of King of England as well. When he joined the crowns in person he also joined the two kingdoms of the island of Britain into one kingdom. Along with a joining of their coat of arms came a joining of their flags in this form, the symbol of the union of the island of Britain. Thus the "Union" part of the name. The "Jack" is also owed to King James as Jack is borrowed from the Latin pronunciation of James. 

Eventually the Union Jack added Saint Patrick's Saltire or Saint Patrick's Cross, a common symbol for Ireland up to the year 1800.

These three were the flags combined to produce the 1801 variant for the flag of the United Kingdom and were a unifying symbol of the most important states of the early British Empire. They carried colors and patterns common throughout Europe at that time and still visible in flags of today. In it we see the colors red, white, and blue as well as the use of crosses. These colors are visible in other European standards such as France, the Netherlands, Norway, Iceland and Finland among others. 

France 

Netherlands:

Poland

The cross is also a common theme that is in several countries in Europe and most likely can be traced to Christian influences around the time of the when flags began being adopted by entire countries. Examples include countries like the previously stated Scandinavian countries as well as Switzerland and others.

Switzerland:

Finally, you arrive at the flag of the UK. You look at it and it appears very unique, but break it apart and you see a history of not only the United Kingdom, but of much of Europe, including shared traditions, values and beliefs. It represents a movement of people and ideas for the better part of a thousand years. But the story doesn't stop there.

Flag of the United Kingdom 1801:

Several countries today trace their roots to times when they were colonies of the British Empire. They feature a Union Jack in the top corner of the flag to denote this with other defining characteristics. Here Australia and New Zealand both feature the Southern Cross constellation:

Australia:

New Zealand:

Other colonies feature the Union Jack, but with the various coat of arms for each respective territory. Now many who still carry this are independent or part of the Commonwealth of Nations, but carry the tradition of their colonial flags. 
Figi:

Bermuda:

Tuvalu:

And a much older one called the Grand Union Flag

Which many believe to be the original to what we now know as...
The Flag of the United States

What is interesting now is how America's influence has influenced the development of other flags in the world through their own colonization and diplomatic efforts. You can see this in how flags of various other nations that were once colonies of America or founded by Americans bare a striking resemblance.

Liberia:

Puerto Rico:

Flag of the former Republic of Texas

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But European influence isn't the only one by far. Now let's look at the flags of much of the Middle East, Northern Africa and parts of Asia.

Flags of the Middle Eastern heritage.

Yemen:

Syria: 

Egypt:

Iraq:

Palestine:

Sudan:

Jordan:

Sahrawi Arab Democratic Republic: (Not in the Middle East, but made up primarily of ethnic Arabs on the West Coast of Africa.)

United Arab Emirates. 

Iran:

Tajikistan (Tajiks are ethnically similar to Iranis):

Libya:

Azerbaijan:

Algeria:

Pakistan:

Turkey:

Tunisia:

Malaysia:

(This one's not what you think. Flag of Malaysia)

From these we can see a few different patterns that center on a few main themes. What you will notice first is the prevalence of the Pan-Arabic flag and colors which is a pattern that has been adapted as a unified symbol of much of the Islamic world. Upon change of regime or independence many nations chose this scheme to show their shared cultural link with Islam. 

Secondly you will notice the pattern of star and crescent, which historically goes back to the Ottoman empire. It was a symbol used by the Ottomans and spread throughout their empire and now can be seen in the flags of Islamic nations spread as far as Pakistan, Turkey and even Malaysia.

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Summary:

Flags are a link to a much larger story about the people who allow them to be their civic and personal banner. A group of people must identify with them to allow them to wave. A flag is a unifying element that binds a group together and separates them from others. It codifies values and builds walls from the outside. They are adopted by those who want to emulate other great nations. They evolve over time, but by looking at them as a whole we see a story. So you can see the effects of colonialism, religion and ethnicity on how different nations have adopted similar flag variants. From Denmark to the UK to Texas, Saudi Arabia, to Turkey to Malaysia flags can serve as a family tree of nations and cultures. They show the movements of people and ideas and are a link to our shared cultural heritages. All this to say there is a lot of reason why they look a lot alike.

Via Don van der Drift.