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.

The above are all suffixes to floating-point numbers. Thus, *5* is five pixels, *5c* is five centimetres, and so on.