This has to be one of the most valuable features to the Mac OS X for those re-installing. One used to need to load original version, which, if you had an old Mac, could have been a few versions back. Then you needed to patch, and then you needed to upgrade through each subsequent MAC OS X versions. Now, Apple has fixed this time consuming effort.
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Users holding Command-Option-R at startup will force a netboot, and will install the latest version of the operating system, up to date with all patches."