Highlight text that extends beyond a certain column.
This mode is meant to be a very light weight, 0 configuration, way to help enforce the 80 column rule. It can be configured for any N-column rule however.
What it looks like enforcing the 80 column rule:
Start it up interactively on the current buffer,
default startup: M-x: column-enforce-mode or by specific rule function M-x: 80-column-rule
or tie it to a major mode:
(add-hook 'c-mode-hook 'column-enforce-mode)
Create preconfigured N-column-rules
(make-column-rule 110) ;; creates function `110-column-rule' for use interactively