In multi-column layouts, the column-rule CSS property specifies a straight line, or "rule", to be drawn between each column. It is a convenient shorthand to avoid setting each of the individual column-rule-* properties separately : column-rule-width, column-rule-style and column-rule-color.

[Webplatform](docs.webplatform.org/wiki/css/properties/column-rule "Webplatform")
[Mozilla docs](https://developer.mozilla.org/en-US/docs/Web/CSS/column-rule "Mozilla")
[Caniuse](http://caniuse.com/#feat=multicolumn "Caniuse")