In the early days of personal computers, operating systems like Windows and MS-DOS used file extensions to figure out what type of files they were and what program could open them.
Modern operating systems like macOS have different ways of figuring out what app is associated with what files but file extensions have stuck with us anyway.
If you’re trying to figure out what format a file is in, the extension is a good place to start. Let’s look at some of the most common file extensions for documents on your PC or Mac.
A Brief History of Document File Extensions
File extensions have been around since before personal computers were commonplace but they started to see widespread use in 1980, with the release of the IBM PC and MS-DOS.
MS-DOS filenames could be up to 8 characters long with a 3-character extension. This became known as the “8.3” filename.
When Apple’s Macintosh computer came on the scene in 1984, it did away with file extensions completely. Instead, it used a type code within the file to identify the format.
Windows 95 made PCs work more like the Mac by getting rid of the 8-character limit and not needing file extensions to identify the format. All versions of macOS and Windows since then haven’t required it but most files still include an extension.
Microsoft Office Documents
Microsoft Office includes some of the most widely-used applications on personal computers – Word, Excel, and PowerPoint. Those document formats are some of the most common of any filetype.
MS Word uses the .DOC file extension to identify its documents. Newer versions of Word use a slightly upgraded version known as .DOCX.
Excel spreadsheets typically use the. XLS extension. As with Word, Microsoft added some features to the format a few versions back, identified with the .XLSX extension.
PowerPoint presentations use the .PPT extension, modified to .PPTX in later versions. Interestingly, that’s one of the few common extensions that are more than 3 characters long.
The Office document formats are widely supported in other applications. When you have a document with one of those file extensions, it doesn’t mean you have to use MS Office to open them.
For example, Google Docs can open .DOC and .DOCX files that were created in Word and vice versa.
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Portable Document Format
In the early 1990s, document formats weren’t exactly universal. If you had an MS Word document, you really needed to open it in Word to see it as intended. While other programs could import those formats, there were usually problems like fonts and layouts not translating across properly.
This made it hard to exchange formatted documents with other users, especially if they were running a different operating system. To remedy this problem, Adobe invented the portable document format, commonly known as PDF.
A PDF file looks the same on any computer, no matter what applications are installed or what operating system it’s running. Because they’re so universal, they’ve become a standard for distributing documents, forms, and other information.
This has led to the opposite problem though. While a PDF can be opened on virtually any computer, it can’t be edited without specialized software. There are ways to extract the text from a PDF document if necessary.
Image File Extensions
Another common type of file, especially on the internet, is image files. There are quite a few different file formats for images but only 3 that you’ll typically see:
- GIF
- PNG
- JPG
GIF files are usually short animations or videos with no sound. PNG and JPG are both image format for photos and other graphics.
JPG is a compressed format so it can take less storage space than PNG but at the cost of some image quality. A high-resolution JPG will still look great but as you crop or zoom into the image, you’ll start to see more noise than with PNG.
Plain Text
Plain text (.TXT) is probably the most universal file format of all. Practically every computer ever made can read text files.
You could create a text file on a new computer running the latest version of Windows or macOS, transfer it to a computer from 40 years ago, and that computer could open the file. The only catch would be finding a way to transfer it since storage media has changed so much.
Common File Extensions On the Web
It’s not obvious to the average person browsing the web but web pages are really just files stored on a server somewhere on the internet. When you load a page in your browser, the file gets transferred to your computer and your browser translates it into the page you see.
The most common file types and extensions on the web is HTML, which uses the .HTML or .HTM extension. You don’t always see these extensions because browsers are smart enough to load the file without being explicitly told to do so.
For example, when you browse to www.google.com, that’s all you see. But your browser is actually loading an “index” file from Google’s server. That file could be named index.html.
Other common web-based file extensions include:
- PHP
- XML
- TXT
- MD
Decoding Common Document File Extensions
These are some of the most common file extensions but it’s far from an exhaustive list. There are lots of unusual and rarely-used extensions that you may come across as well.
If you find yourself with an unknown type of file that you need to open, the best way to track it down it to search for the file extension on Google. You’ll find lots of helpful pages with long lists of file types and what apps they work with.
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