The column-count property specifies the number of columns desired, and the maximum number of columns allowed. The default value of auto means that the element has one column. Our leftmost articles are broken into three columns, and the article below the ad blocks has two columns:

#primary article .content {
-webkit-column-count: 3;
-moz-column-count: 3;
column-count: 3;

#tertiary article .content {
-webkit-column-count: 2;
-moz-column-count: 2;
column-count: 2;

The columns will have a small gap between them. The total width of the columns combined with the gaps will take up 100% of the width of the element.
Related Tutorial
5. columns