You might have noticed that my tablet UI proposal is missing rulers. That’s because I wanted to make rulers contextual: if a user inserts a tab or taps within the empty space where a tab is, a tab handle and a horizontal ruler should appear, allowing the user to adjust the tab width. Or if the user inserts a floating object (image, table, shape, …), rulers should appear. Or if the user selects a page, he should be able to adjust the margins.
I also thought that maybe there could be a single “ruler mode”, where the user is able to adjust all the lengths and widths within the page (margins, column widths, indents, tabs). I’m wondering: should this mode be triggered by selecting the page? Then the user could get to this mode much faster.
Possible page selection modes
or 
With the ruler shown, the user would able to edit the units it uses. With an object selected, the user would also have the ability to type in the object’s distance from the margins. Here’s a mockup of what it could look like for a selected tab:
What do you think?
