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A double-byte character is a character value represented by two bytes in a particular double-byte character set (DBCS). The term describes an encoding, not a rule that every character in a text string—or every Unicode character—uses two bytes.
What does “double byte” mean in computing?
In a DBCS, some character values use one byte and others use two. Microsoft Learn defines a DBCS as an “extended single-byte character set” implemented as a code page. English letters and digits may have single-byte values, while many East Asian characters may use two-byte values. Which characters use which values depends on the specific code page.
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A two-byte character in a multibyte character set can be encoded as a lead byte followed by a trail byte. The valid byte ranges and their interpretation are specific to the encoding. A byte sequence cannot reliably be identified as a character without knowing which code page or encoding applies. Microsoft Learn: Double-Byte Character Sets and Microsoft Learn: Character Sets
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No. A DBCS can mix single-byte and double-byte values. “Double-byte” describes the values that take two bytes, not the length of every character in the set or every string encoded with it. A string’s total byte length depends on its characters and the encoding used.
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Are double-byte characters the same as Unicode characters?
No. Unicode defines a repertoire of characters and has multiple encoding forms; it is not itself a two-byte encoding. According to the Unicode Consortium glossary, UTF-8 uses one to four bytes per Unicode character, while UTF-16 uses two or four bytes per Unicode character. Therefore, neither “Unicode character” nor “UTF-16 character” means that the character always occupies exactly two bytes. Unicode Consortium: Glossary
Keep the encoding in view when discussing byte length: a character’s byte representation depends on the chosen encoding form, and a file or protocol’s declared encoding determines how its bytes should be read.
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Are “double-byte” and “fullwidth” the same thing?
No. “Double-byte” concerns encoded storage; “fullwidth” concerns rendered width. In East Asian text, fullwidth glyphs typically occupy two display cells, while halfwidth glyphs occupy one. Those display-cell descriptions do not establish how many bytes a character takes in a particular encoding. Unicode Standard chapter 18 discusses this terminology in the context of East Asian coding systems. Unicode Standard, Chapter 18: East Asia
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| Term | What it describes | What to remember |
|---|---|---|
| Double-byte | A character value represented using two bytes in a particular DBCS | A DBCS may also contain single-byte values. |
| UTF-8 or UTF-16 | Unicode encoding forms and their byte representations | UTF-8 uses one to four bytes and UTF-16 two or four bytes per Unicode character. |
| Fullwidth or halfwidth | Typical display-cell width in East Asian contexts | Display width is not a byte count. |
When does the distinction matter?
- Reading legacy data: Identify the file’s or protocol’s code page before interpreting byte sequences. A byte that serves as a lead byte in one encoding may have a different meaning in another.
- Estimating storage or string length: Do not infer byte length from a character’s language, appearance, or display width. Use the actual encoding.
- Developing Windows applications: Microsoft recommends Unicode for new Windows applications to avoid inconsistencies among code pages and ease localization. A legacy protocol may still require a particular DBCS code page. Microsoft Learn: Double-Byte Character Sets
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