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John Hudson said:
Georg, I don't think the averageWidth field of the OS/2 table is actually used by software handling layout for monospaced fonts. Rather, the width of a representative glyph (e.g. /space/) is cached, and then used to advance the width of all painted glyphs, regardless of their encoding. This exlains the behaviour you see in e.g. InDesign if you try to make mark glyphs zero-width in a monospaced font: they end up getting assigned the same advance width as the others in line layout.