Like the next five icons, this function acts as a switch: A click
on it activates the associated function (and the icon will be
displayed as selected in inverse video), a further click switches it
off (icon unselected). If it is switched on, so-called magnetic
horizontal guidelines will cause the top and bottom edges of the frame
to snap to the nearest guidelines when creating or resizing a frame.
Thus the frame will stretch over the whole area from the guideline
above its top edge to the guideline below the bottom edge. The borders
of the page will also act as guidelines. When you move a frame, the
top edge of the frame will snap upwards or downwards to the nearest
horizontal guideline.
