To use centimeters in the ruler instead of inches in Cocoa applications, in the file .GlobalPreferences.plist located in ~/Library/Preferences/ add:
<key>NSMeasurementUnit</key>To see the effect, open TextEdit, make sure you have a Rich Text format document, and then hit command-R to show the ruler.
<string>Centimeters</string>
Mac OS X Hints
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