Editing Big5 (section)

===The Matching SBCS===
In practice, Big5 cannot be used without a matching SBCS; this is mostly to do with a compatibility reason. However, as in the case of other CJK DBCS character sets, the SBCS to use has never been specified. Big5 has always been defined as a DBCS, though when used it must be paired with a suitable, ''unspecified'' SBCS and therefore used as what some people call a [[Variable-width encoding|MBCS]]; nevertheless, Big5 by itself, as defined, is strictly a DBCS.

The SBCS to use being unspecified implies that the SBCS used can theoretically vary from system to system. Nowadays, ASCII is the only possible SBCS one would use. However, in old [[MS-DOS|DOS]]-based systems, [[code page 437]]—with its extra special symbols in the control code area including position 127—was much more common. Yet, on a Macintosh system with the Chinese Language Kit, or on a Unix system running the cxterm terminal emulator, the SBCS paired with Big5 would not be code page 437.

Outside the valid range of Big5, the old DOS-based systems would routinely interpret things according to the SBCS that is paired with Big5 on that system. In such systems, characters 127 to 160, for example, were very likely not avoided because they would produce invalid Big5, but used because they would be valid characters in code page 437.

The modern characterization of Big5 as an MBCS consisting of the DBCS of Big5 plus the SBCS of ASCII is therefore historically incorrect and potentially flawed, as the choice of the matching SBCS was, and theoretically still is, quite independent of the flavour of Big5 being used.