Grid system - a tool for layout
A "grid" is a tool for creating a consistent and balanced layout. It consists of a number of guides (horizontal and / or vertical) and serves as a template where you place text, image and other objects along the grid lines.
The base consists of vertical lines that create a number of columns and spaces.
Used correctly, the grid system makes it easy to find a "red thread" in the layout. Not just in one printed matter, but between several devices.