Shared Tables and Unique Tables

Any tables which have identical data for two or more fonts may be shared. It is perfectly acceptable for a table to be shared among only a subset of the fonts in a TTC. For maximum space efficiency, whenever possible, tables should be constructed so they can be shared.

The space efficiencies of a TTC are greatest when the TTC is constructed with a single 'glyf' table to be shared by all fonts. A shared 'glyf' table means that many other tables which coordinate closely with 'glyf' can probably also be shared: 'hmtx', 'loca', 'EBLC', 'EBDT', 'maxp', 'vmtx', and so on.

A table cannot be shared if font functionality requires it to hold different values for each font. Tables that must be unique to each font are those that are used by the system in identifying the font and its character set. These include 'cmap', 'name', and 'OS/2'.

The sets of shared and unique tables will vary from one TTC to another, depending on font features and the production methods used.