File Extensions

File extensions are used to indicate the format of files so operating systems know what to do with them. Learn why there are so many different file extensions. Also learn about some of the most widely used file extensions.

File Extensions

So, you're working on an essay for school, and you use a word processing application to enter your text. You save this file on your computer and call it 'MyFirstReport.' Then you open up your e-mail to send it to your instructor. You attach the report to your e-mail, but when it shows up in your e-mail message, it is called 'MyFirstReport.doc.' Wait a minute - you did not type the .doc part. Where did this come from?

You probably know the answer. The .doc part of the file means that the file was created using Microsoft Word, and it was automatically added to your file name by the software. In this case, doc is short for - you guessed it - document. This is called a file name extension, or file extension for short.

A file extension is part of the file name and uniquely identifies the type of file, also referred to as the format. When your instructor receives your e-mail and sees your file with a .doc extension, she knows that you are sending her a Word document. But, not all file extensions are quite as intuitive. Quick, what does .qbe stand for? Hmm…

Why File Extensions?

File extensions are used so that the operating system, or OS, of a computer can recognize the file type. When your OS sees a file with a .doc extension, it knows that this file is the native format of Microsoft Word. So, when you double-click on a file, your OS will automatically launch the correct software application and open up the file in this application.

Most file extensions consist of three characters, but the number of characters can vary. For example, the file extension .py only has two characters. This file extension is used for files in the widely used Python programming language. Letters are most common, but some file extensions use numbers or special characters. For example, the file extension .wp5 is used for files created using version 5 of the word processing application WordPerfect.

There are hundreds of file extensions - from .a to .z. And then there are special characters and numbers, too. Why so many? Some of this results from the fact that different files can contain very different data. For example, a .txt file contains text, while a .mus file contains music. So, file extensions can be data-contents specific.

However, if that was the only reason, you wouldn't need hundreds of different ones. So, a second reason for so many different file extensions is that different software applications organize similar data in different ways. For example, documents created using word processing software use mostly text, but Microsoft Word saves files as a .doc, while WordPerfect saves files as a .wp. Each application is just a little different in how the contents of a document are stored in a file. File extensions can therefore be software-application specific.

There are some very specific technical reasons for this. For example, digital photographs require lots of storage. Clever algorithms have been developed to compress the data, and there a number of different algorithms - each corresponding to a different file extension. So, you can store a digital image as a .jpg, .png or .tif, and each one represents a different compression algorithm for the same type of data. These formats are not specific to a particular software program, and most photo editing software can open all these different formats. File extensions can therefore be algorithm specific.

Some formats are also very specific to a single operating system. For example, some multimedia files may only play on a computer with Windows and some only on a Mac. Most formats, however, can be used on multiple platforms. Finally, different companies may have developed proprietary format. So, while some formats are almost the same in terms of functionality, different file extensions are used to recognize their proprietary nature.

Some Key Extensions

There is no need to remember all the different file extensions. Remember that your computer's OS recognizes the file extensions and knows which applications to use for which format. However, it is good to be aware of some of the most widely used ones.

File extensions like .exe represent executable files. In other words, they can run by themselves and don't need to be opened by a software application. When you download software, it often comes as a .exe file. When you double-click the file, the software installation starts. You should be very careful with .exe files, especially if you receive them by e-mail. Operating systems have difficulty determining exactly what is inside a .exe file, and computer hackers often hide viruses inside these executable files. Unsuspecting users run the executable file and end up with an infected computer.

One of the most widely used productivity suites is Microsoft Office. Each software application has its own file extension, including a .doc (Word), a .xls (Excel) and .ppt (PowerPoint). In the most recent version of these software applications, the format was updated and the extensions changed to .docx, .xlsx and .pptx, respectively.