What is a redline?

Updated 30 September 2026

A redline is a version of a document that shows what changed from a previous version: deleted text struck through, new text underlined, usually in colour. The name comes from the red ink editors once used on paper. In some industries, particularly legal, the same thing is called a blackline.

Redlines and tracked changes

In Word, a redline is usually a document with tracked changes. Each insertion, deletion and formatting change is recorded with an author and date, and a reviewer can step through them and accept or reject each one. That makes a redline more useful than a static PDF markup: the reviewer can act on it.

Two ways to get a redline

• Track changes while editing. Turn on Track Changes before making edits, and Word records them as you go. This only works if everyone remembers to leave it on.
• Generate it from two finished versions. If you have the original and the revised file, a comparison tool can work out the differences and write them as tracked changes. This catches edits made with tracking switched off.

Making a redline from two versions

Word's Review → Compare produces a compared document with tracked changes. RevisionProof does the same in your browser: load the original and the revised .docx, check the differences on screen, then download a redline .docx. Open it in Word and every difference is there as a tracked change, including formatting changes. Accepting all changes gives you the revised document; rejecting them all gives you the original text back.

When to use a redline

• Sending a revised contract back to the other side.
• Asking reviewers to approve changes to an SOP, policy or specification.
• Showing an auditor or customer exactly what changed between issues.