Rescaling the ruler isn't possible in Word 2003. However, when you drag a tab-stop across the ruler, it movers in very small increments. In Word 2010 & later (not sure about 2007) it appears to jump from one ruler increment to the next.

Probably a design decision in response to feedback from people who found the old approach made it unnecessarily difficult to use this method to align tab-stops in different paragraphs.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]