In Quark the position of objects placed on the page is measured by the position of the ruler origin. When you start a new project the ruler origin starts as default from the top left hand side of the page. This means that the x and y coordinates in the measurements palette show the position of a selected object from the top left of the page. However, when setting some documents it can be necessary to change the ruler origins so that objects can be measured from a different point on the page. This can be done by locating the ruler origin currently at the top left of the page. It appears in between the two rulers as show here.
It is then possible to click on the ruler origin and drag it to a new position on the page. You will see a cross appear on the page as in the picture above to show that the ruler origin is moving. Once you have done this you will notice that the ruler no longer measures from the top left hand side of the page, but now starts at 0 from your new origin point. To reset it to the default setting simply double click on the box where the two rulers join and its back to measuring from the top left of the page.