Moving the ruler zero point
It is often easier to measure objects if you align the 0,0 point of the rulers with the object.
To move the ruler zero point
Drag the square shown at the intersection of the two rulers to the new 0,0 point. This also moves the grid origin so the divisions on the ruler always align with the grid.
Tips
Moving the zero point on the rulers also moves the grid origin.
You can move the origin more accurately in the Grid and Ruler Options.
To reset the origin to the bottom left corner of the page, double click on the square at the intersection of the two rulers.
If you turn on object snapping (Window menu) you can set the ruler origin exactly on the edge or corner of an object.