About OS / 2 Metric: the fonts I have considered (Typo Ascent, Typo Descent, Line Gap etc) never coincide with those suggested by FontLab. Moreover the sum of Typo Ascent and Typo Descent should conventionally be equal to 1000 em for PS. Now, if these values are not respected, what actually happens to the font?
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Family Blues try to harmonize the rendering of a family of fonts, by making sure alignment zones with the same name round to the same height at the same size, even if they are slightly different (and would occasionally round to a different height) between the various styles.
Vertical metrics determine the size of the invisible rectangle that the letters sit upon, and their vertical position within this rectangle. Those values should most likely be the same throughout a family of fonts.