I've previously written about dynamic forms in Drupal, using #states to show or hide input boxes depending on other inputs. Since then, Drupal 7 and 8 have both got the ability to combine conditions with OR and XOR operators. This makes it possible to apply changes when a form element's value does not equal something, which is not obvious at all.
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#Drupal Planet
#Forms
##states
#Drupal 8
#dynamic
#conditions
#jquery
#dependencies
#not equal