Find out which macOS your Mac is using - Apple Support
OS X El Capitan Available to Download Now for All Mac Users | OSXDaily
OS X El Capitan review: A swifter, more streamlined take on OS X - CNET
Amazon.com: Bootable DVD DL for Mac OS X 10.11 El Capitan Full OS Install Reinstall Recovery Upgrade
OS X El Capitan on Unsupported Macs | MacRumors Forums
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Identify your MacBook Air model - Apple Support
OS X El Capitan - Wikipedia
How to reinstall OS X el Capitan on Macbook Air or Pro - YouTube
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Identify your MacBook Air model - Apple Support
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Identify your MacBook Pro model - Apple Support
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OS X 10.11.5 El Capitan Update Available for Mac | OSXDaily