In CJK Computing
The term DBCS traditionally refers to a character set where each graphic character is encoded in two bytes. The DBCS always has lead bytes with the most significant bit set (i.e., being greater than 7 bits), and is always paired up with a single-byte character-set (SBCS). Furthermore, for the practical reason of maintaining compatibility with unmodified, off-the-shelf software, the SBCS is associated with halfwidth characters and the DBCS with fullwidth characters.
Sometimes, the use of the term "DBCS" can imply an underlying structure that does not comply with ISO 2022. For example, "DBCS" can sometimes mean a double-byte encoding that is specifically not EUC.
Note that this original meaning of DBCS is different from what some consider correct usage today. Some insist that these character sets be properly called either multi-byte character sets (MBCS) or variable-width encodings because character sets like EUC-JP, EUC-TW, GB18030 and UTF-8 use more than 2 bytes for some characters, and they support 1 byte for some other characters.
Read more about this topic: DBCS