What is exactly racking issue?

What is exactly racking issue? I am hearing a lot with H-bot and CoreXY designs?

Pls explain with pics.

Racking tends to make box structures into a parallelogram.

The engineering solution is typically angled struts to resist the racking.
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Here’s a good explanation of the issue: http://www.doublejumpelectric.com/projects/core_xy/2014-07-15-core_xy/

@Paul_Gross @Ryan_Carlyle Thanks for the explantion.