With this icon you can position horizontal guidelines.
This and the three other icons are mutually exclusive: Only one of them can be selected at any one time. If this icon is selected, the mouse cursor appears as a cross in the layout window. Every click of the left mouse button now sets a horizontal guideline at the current position.
Guidelines are displayed in Calamus chain-dotted. To position a guideline as accurately as possible, a high magnification factor is recommended. The Coordinate display then shows you the exact position of the mouse cursor on the page.
As for vertical guidelines, a guideline will be placed at the current cursor position, except that in this case it will be a horizontal one.