7. The Menus / 7.3 Functions in the View Menu
Show Ruler turns the rulers in the document window on or off. The default unit for the rulers is "pixels", but by right-clicking in one of the rulers the unit can be changed.
The rulers are always showing the mouse position with a small stroke. This eases positioning the mouse.
The rulers have an additional task: they are used for creating guides. By clicking in a ruler and dragging a guide line is created that can be moved to the intended position. An already created guide can be modified and deleted with the layer tool (see chapter 6.2.1). Guide lines can be edited by using Guides (see chapter 4.37), too.
The small field between the two rulers can be used to set the origin of the rulers. Clicking and dragging allows to move the origin to the required position, double-clicking allows to enter the position numerically.