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

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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.

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