Richard Cook explained the Above the Line / Below the Line Framework in 4 minutes at Velocity NYC 2017. Once you adopt this perspective, once you step away from the idea that below the line is the thing you're dealing with and understand that you're really working above the line all sorts of things change. youtube 
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David Woods & Richard Cook. Stella Report. Velocity NYC 2017. 1m35s—5m35s
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a typical view of the software system serving customers
a view of the system which includes the deploy tools, version control, testing, monitoring
The part of the system most software people miss. This Drs Woods and Cook showing all us fish what water is.
This is the inclusive view of the system divided by the line of representation. Humans above, computers below.
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At IT Revolution conference in 2018, Richard Cook explained the above the line / below the line framework, clarifying complexity, mental models and emphasizing that failure is the normal state. The study of incidents is the revealing thing about how systems actually work. See Working in the Cyclone.
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Richard Cook explained the above the line / below the line framework for understanding systems at IT Revolution in 2018. ~7 minutes. youtube
See another telling by John Allspaw from DevOps Enterprise Summit 2017. Human Performance in Systems
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6 min. John Allspaw. DevOps Enterprise Summit 2017. youtube