In Word, spaces are a consistent width for text that is not justified. That width is determined by the font file and whatever point size, etc, you've specified (it is possible, but unusual, to have different-point-sized spaces). When justification is used, however, the spaces expand/contract so the words fill each line. Do note too that the on-screen presentation can vary, depending on whether you've checked the 'Optimize character positioning for layout rather than readability' option and that Word will try to represent the positioning to match the printer capabilities reported by the attached printer driver.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]