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Furthermore, it is strongly recommended that implementers do not rely on this value for computing layout for lines of text, especially for cases where complex scripts are used. The value for xAvgCharWidth is calculated by obtaining the arithmetic average of the width of all non-zero width glyphs in the font. The Average Character Width parameter specifies the arithmetic average of the escapement (width) of all non-zero width glyphs in the font. The version number allows for identification of the precise contents and layout for the OS/2 table. The version number for the OS/2 table: 0x0000 to 0x0005. This section provides details covering all versions of the OS/2 table. Applications should check the table length for a version 0 OS/2 table before reading these fields. Some legacy TrueType fonts may have been built with a shortened version 0 OS/2 table.

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Note: Documentation for OS/2 version 0 in Apple’s TrueType Reference Manual stops at the usLastCharIndex field and does not include the last five fields of the table as it was defined by Microsoft. The format of version 0 is as follows: Type Version 0 was defined in TrueType revision 1.5. The format of version 1 is as follows: Type Version 1 has five fewer fields than version 2, and two additional fields beyond those in version 0. Version 1 was defined in TrueType revision 1.66.

The format of version 2 is identical to the format for version 4, given above. Version 2 has the same fields as in version 3, and five additional fields beyond those in version 1. The format of version 3 is identical to the format for version 4, given above. Although new fields were not added beyond those in version 2, the specification of certain fields was revised to reflect changes in Unicode 3.2. Version 3 has the same fields as in version 4, and as in version 2. The format of version 4 is as follows: Type Although new fields were not added beyond those in version 3, the specification of certain fields was revised. Version 4 has two fewer fields than version 5, and the same fields as in version 3. The format of version 5 is as follows: Type Version 5 has two additional fields beyond those in version 4. All versions are supported, but use of version 4 or later is strongly recommended. Six versions of the OS/2 table have been defined: versions 0 to 5. The OS/2 table consists of a set of metrics and other data that are required in OpenType fonts.

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