Imagine the customs area at a large international airport. Hundreds of people are lined up, suitcases at their sides, passports in hand. Those travelers might include citizens from dozens of different countries. Their passports may not look the same at first glance, but the customs agents who inspect them know that they're more similar than they are different. Although each country is allowed some leeway when designing its passport, U.S. passports and those issued by other countries serve the same purpose.

An Overview of Passports and Their Purposes

In short, a passport is a travel document issued to citizens of a country by that country's government. The passport holder (the person whose name and picture appear inside) may use the passport to prove both identity and nationality. An American citizen, for example, can use a U.S. passport to prove that he is: a) who he says he is and; b) a legal citizen of the United States. Proving those points to customs agents at a country's border is a requirement to be allowed into the country. Travelers must show their passports when they arrive at any border, even when they're trying to reenter their own countries.

Each country's government issues its own passport design, but all passports must conform to the standards set by the International Civil Aviation Organization, or ICAO. It's an agency of the United Nations that works with UN member nations to reach consensus about aviation practices to create industry standards for things like aircraft emissions and airport security. The ICAO also sets standards for what it calls Machine Readable Travel Documents, or MRTDs, a category that includes passports. So while each country has some flexibility in creating its own passport, they must all share some of the same features.

What All Passports Must Include

The ICAO's guidelines for passport design are comprehensive. They require that passports measure 88 by 125 millimeters in size and include at least eight pages, and they spell out the way that passport pages should be laid out and the information that must be included inside. The holder's name, nationality, image and signature must be included in the Visual Inspection Zone of the passport – basically, the pages that a customs agent flips to when screening a traveler. All passport photos must conform to the ICAO's guidelines too. Images must be closeups, show only the subject, and be less than six months old at the time that the passport is processed, among other requirements.

How U.S. Passports Differ From Others

The color of the cover is one of the most obvious differences between passports. All countries use red, black, blue or green for standard passports, although there's a lot of variation in shades. American passports are a deep blue, for example, while Fiji passports are sky blue. The name of the issuing country is printed on the cover of each passport. The country's seal might be included, too.

Many countries customize the pages of their passports in ways that celebrate the country's history and culture. The first page of an American passport displays a quote from Abraham Lincoln and a message from the Secretary of State. The rest of the pages in a standard American passport have classically American images printed in the backgrounds. The data pages – where the holder's name, picture, other identifying details and signature are located – include an image of an American flag and bald eagle. The visa pages, where customs agents place stamps each time the holder enters a new country, depict images of Mount Rushmore, the Liberty Bell and the Statue of Liberty.

In international passports, those pages may be printed with images that are significant to the history of the specific country. For example, the signature page of a Canadian passport displays an image of the country's flag and maple leaves.