Paste your original text on the left and the revised version on the right to see every line that changed.
Comparing two versions of a document by eye is one of the more tedious editing tasks there is — a contract redline, a blog post before and after an editor's pass, or two drafts of code comments can look nearly identical at a glance while hiding a handful of meaningful changes. A diff checker exists to remove that guesswork by comparing both texts programmatically and reporting exactly which lines differ.
This tool works line by line: any line that appears in your original text but not in the revised version is marked as removed, any line that's new to the revised version is marked as added, and lines that appear in both are left unstyled so your eye is drawn straight to what actually changed. This line-based approach is the same method used by most code version control tools (like the diff view in Git), just applied to plain text instead of source code.
It's commonly used by writers comparing draft revisions, students checking what an editor or professor changed in feedback, developers comparing configuration files or notes, and legal or contract reviewers spotting clause changes between document versions. Because the comparison runs entirely in your browser, it's also safe to use on confidential or unpublished material — nothing is uploaded to a server in the process.