Missing character glyphs in fonts have a wide variance of appearances,
just like LATIN CAPITAL LETTER A glyphs.
If it will help to understand this issue, here is how it works from
a font perspective.
The missing glyph is the first glyph in any font. This is mapped to
U+0000 and the system correctly substitutes the glyph mapped to
U+0000 any time a font being used lacks an outline for a called
character. (Unless the application engages in arbitrary font-
switching, which is another story...)
The font specs say that the missing glyph should look like a
hollow square. Actual appearances of glyphs in fonts are always
left up to the font developers.
Since the glyph is mapped to null (0000) and looks like a box, we
often call them null boxes.
More detailed information about how the missing glyph is
inserted can be found in the font specs for the cmap table:
http://www.microsoft.com/typography/otspec/cmap.htm
Best regards,
James Kass.
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