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This tool that's distributed with texlive has no actual documentation for the bbox parameter. It just says that

--bbox "<left> <bottom> <right> <top>"

What units do these number follow? Say I have a letter sized paper, what will be the intervals on which these numbers are defined? Also is the right coordinate computed from the left or right margin? Same with top.

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These coordinates are referenced as bp (or big points), which is different from TeX's pt. More specifically, 1in is equivalent to 72bp or 72.27pt.

A letter-sized paper (8.5in x 11in) consists of 612bp x 792bp, typically with coordinates gives as (0,0) and (612,792). Since these are coordinates, they are not calculated from one another. That is, coordinates of the form (100,100) and (712,892) would still represent a letter-sized paper, while (100,100) and (612,792) represents a sheet of paper that has dimensions 7.11in x 9.61in.

Conversions between units is available, as an example, from Is there a command to convert cm to bp?

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Wikipdia says 1 point = 1/72 inch, en.wikipedia.org/wiki/Point_%28typography%29. What is the difference and relation between point and big point? Does "pt" in TeX not mean "point" in Wikipedia? – Tim Nov 29 '14 at 13:29
@Tim: Yes, there is a difference. Wikipedia's point refers to a big point in TeX. TeX's point is a little smaller. See Why does a TeX point differ from a Desktop Publishing point? – Werner Nov 29 '14 at 15:55

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