If you use Microsoft Word, you have likely encountered both .doc and .docx files. While both open in Word and look identical to the user, they are fundamentally different under the hood.

The transition from DOC to DOCX happened back in 2007, but legacy files are still floating around. Here is why the "X" is so important.

What is a DOC File?

The DOC format was the default proprietary format for Microsoft Word from its inception up until Word 2003.

It is a binary format. This means all the text, formatting, images, and structural data are interwoven into a single, complex, closed file structure. Because it was proprietary, it was notoriously difficult for non-Microsoft software (like Apple Pages, Google Docs, or OpenOffice) to open DOC files perfectly without formatting errors.

What is a DOCX File?

Starting with Word 2007, Microsoft introduced the DOCX format. The "X" stands for XML (Extensible Markup Language).

Unlike the monolithic binary structure of a DOC file, a DOCX file is actually a zipped folder.

The "Zip" Secret

If you take a document.docx file, rename it to document.zip, and extract it, you will see a folder structure containing XML files (which hold the text and formatting) and a "media" folder (which holds any images you inserted into the document).

Why DOCX is Superior

The shift to XML brought three massive improvements:

1. Much Smaller File Sizes

Because a DOCX file is essentially a zipped archive, it is heavily compressed. A DOCX file is often 50% to 75% smaller than the exact same document saved as a DOC file. This saves storage space and makes emailing large reports much faster.

2. Significantly Better Security

The old DOC binary format was highly susceptible to macro viruses. Malicious code could easily be hidden within the complex binary structure.

DOCX files, by design, cannot execute macros. (If you need macros in a modern Word document, you must save it as a specific .docm file). This makes DOCX inherently safer to open when received via email.

3. Far Less Prone to Corruption

If a few bytes of data in a DOC file became corrupted, the entire file would often refuse to open.

Because DOCX stores text, formatting, and images separately, a corrupted image won't necessarily destroy the text data. You can often still recover the XML text files even if the overall archive is damaged. Furthermore, because XML is an open standard, DOCX files are perfectly compatible with Google Docs, LibreOffice, and almost all third-party word processors.

Do You Still Need DOC?

In 2026, there is virtually no reason to use the older DOC format. The only exception is if you need to send a document to someone running a very old computer with Microsoft Word 2003 or earlier installed, without the compatibility pack.

How to Convert DOC to DOCX

If you have an archive of old DOC files, converting them to DOCX will save you disk space and future-proof your documents.

  1. In Microsoft Word: Open the DOC file. Go to File > Info > Convert. This upgrades the file to the newest format.
  2. Online: If you don't have Word installed, you can use ToolzStack's Document Converter to batch convert older .doc files into the modern .docx standard.

Related guides: PDF vs DOCX · What is a DOCX File?