column-gap

The column-gap property specifies the width of the space between columns:

snippet
#primary article .content,
#tertiary article .content {
-webkit-column-gap: 10px;
-moz-column-gap: 10px;
column-gap: 10px;
}

Width should be mentioned in length units, such as ems or pixels, or normal.It’s up to the browser to determine what normal means, but the spec suggests 1em. Our column gaps is given as 10px wide.
Related Tutorial
5. columns