This blog post introduces a small command line utility (OpenXmlDiff.Exe, code attached to this page) that compares two Open XML documents and produces a textual report of the differences in markup between them. This utility was born out of sheer frustration. I've been needing this utility for months. I've always accomplished my goals without it, but it would have been a time-saver several times in the past. I've heard rumors of this existing elsewhere, but it has always eluded me. Using this utility, we can create two documents with a very small difference between them, and see the exact changes that the difference caused. Sometimes this alone is enough to explain the markup. But if further explanation is necessary, the diff makes it easy to find the relevant places in the Open XML specification. For our purposes, the currently published Ecma 376 specification works just fine.