Up to 1,000 lines
Line by line
The two texts are lined up by longest common subsequence — the same idea diff and git use. Lines that only exist on one side are marked added or removed; lines present in both are shown unchanged so you keep your bearings.
Whitespace counts
A line that differs only by a trailing space is still a different line. That is usually what you want when comparing config or data; it is occasionally surprising when comparing prose.
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Frequently asked questions
Q. Is there a size limit?
A thousand lines per side. Beyond that the comparison gets slow in a browser, and a real diff tool will serve you better.
Q. Does it compare within a line?
No, the unit is the whole line. A one-character change shows as one line removed and one added.
Q. Is anything uploaded?
No. Both texts stay in the page.