What it shows
Paste the original on the left and the changed version on the right. The comparison runs as you type: every line is marked as added, removed, or left alone, and a line that was edited is shown against the line it replaced, with the words that actually changed picked out on both sides.
On a wide screen the two versions sit side by side, each with its own line numbers, so you can read across a row and see the before and the after together. On a narrow screen — and on the Unified layout, if you prefer it — the same comparison becomes a single column with a + or − against each line.
Finding the change you are looking for
Two versions of a real document are mostly identical, and the differences are the point. Three things keep them findable:
- The arrows beside the counts step from one difference to the next, and the counter tells you where you are — 3 of 17. A five-line replacement counts as one place to look at, not five.
- Long stretches of untouched text fold up into a single line you can click to open, so a one-word change in a long contract does not arrive buried in forty screens of agreement.
- Turning off unchanged lines reduces the result to the differences alone, with a marked gap wherever text was skipped and the original line numbers still attached.
The options
Ignore decides what counts as a difference:
- Spacing — indentation, trailing spaces and runs of spaces inside a line stop mattering. On by default, because a stray trailing space is almost never the difference you are hunting for.
- Case — treats Monday and monday as the same line.
- Punctuation — commas, quotes and dashes stop counting. Useful when comparing text that has been through a system that rewrites quotation marks.
- Blank lines — leaves empty lines out of the comparison entirely, while the line numbers on screen still refer to the real files.
Show decides how much of the result you see: unchanged lines, line numbers, and the word-level marks inside an edited line. All three start on.
What it is good for
- Checking exactly what an editor changed in your draft, down to the word
- Finding the one line that differs between two config files
- Comparing two exports of a list to see what was added this week
- Reading a redline of a contract clause without a word processor
- Confirming a copy-paste really did copy everything
How the comparison works
It computes the longest common subsequence of the two line lists — the longest ordering of lines that appears in both — and treats everything outside it as added or removed. That is the same approach behind the diff command, and it is why a paragraph inserted in the middle shows as one insertion rather than shifting every line after it.
Inside a block of differences, a removal and the addition that took its place are put on the same row, and the same subsequence method is run again over the words to mark what changed. That second pass only runs when the two lines are recognisably the same line edited; two genuinely different lines are left plain rather than sprayed with marks that mean nothing.
Very large inputs are refused rather than freezing the tab: the work grows with the two documents multiplied together, so a pair of very long files would lock the browser. The matching beginning and end are stripped first, which is why two long drafts that differ in one paragraph compare instantly.
Common questions
Is my text uploaded?
No. The comparison runs in this page. Neither version is transmitted, which is worth knowing if you are diffing a contract or a config file with credentials in it.
Can it highlight changes within a line?
Yes. When a line has been edited rather than replaced, the changed words are marked on both sides — the removed wording on the left, the new wording on the right. Turn it off under Show if you would rather see whole lines.
Can I keep the result?
The copy and download buttons hand over the comparison as plain text, in the familiar form with - against removed lines and + against added ones, which pastes into an email or an issue without further work.
Does it handle different line endings?
Yes. Windows and Unix line endings are both understood, and a file that mixes them will not show every line as changed.
