TrueType outline fonts should have the following four glyphs at the beginning of a font. These were listed in Apple's original TrueType specification. These glyphs are recommended to allow for the same version of the font to work on both Windows and Macintosh.
Glyph 0 is the .notdef (missing character) glyph.
Glyph 1 is the .null glyph; it has no contours and zero advance width.
Glyph 2 is the nonmarkingreturn glyph; it has no contours and positive advance width.
Glyph 3 is the space (and no-break space) glyph; it has no contours and positive advance width.
* For the Macintosh platform, all unmapped characters are mapped to the first glyph.
Glyph 2 and 3 should have the same advance width.
The .notdef glyph is very important for providing the user feedback that a glyph is not found in the font. This glyph should not be left without an outline as the user will only see what looks like a space if a glyph is missing and not be aware of the active font's limitation.
It is recommended that the shape of the .notdef glyph be either an empty rectangle, a rectangle with a question mark inside of it, or a rectangle with an "X". Creative shapes, like swirls or other symbols, may not be recognized by users as indicating that a glyph is missing from the font and is not being displayed at that location.
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