These are the valid screen distances, according to the man page for Tk_GetPixelsFromObj, Tk_GetPixels, Tk_GetMMFromObj, Tk_GetScreenMM:

   * '''<none>''' The number specifies a distance in pixels. 
   * '''c''' The number specifies a distance in centimeters on the screen. 
   * '''i''' The number specifies a distance in inches on the screen. 
   * '''m''' The number specifies a distance in millimeters on the screen. 
   * '''p''' The number specifies a distance in printer's points (1/72 inch) on the screen.