ASCII 21
ASCII 21 is Negative Acknowledge — the control character NAK.
Every representation
| Character | NAK (not printable) |
| Name | Negative Acknowledge |
| Decimal | 21 |
| Hexadecimal | 0x15 |
| Octal | 0025 |
| Binary | 00010101 |
| HTML entity |  |
| Escape sequence | \x15 |
| Unicode | U+0015 |
| Keyboard | Ctrl+U |
| Group | Control character |
What it is for
A negative reply — the message did not arrive intact, please resend.
Where the number comes from
Codes 0 to 31 were reserved for control: instructions to a teleprinter rather than characters to print. Code 21 is Ctrl+U because the Ctrl key was originally wired to clear the top two bits of whatever you typed, mapping the letter U at 85 down to 21.
Frequently asked questions
What is ASCII 21?
Negative Acknowledge (NAK), a control character. In hexadecimal it is 0x15, in octal 0025, and in binary 00010101.
How do I type ASCII 21?
Press Ctrl+U in a terminal. In a string literal you can write it as \\x15 in most languages.
Is ASCII 21 printable?
No. Codes 0 to 31 and 127 are control characters — they were instructions to a teleprinter rather than marks on paper, and most terminals show nothing for them.
Nearby codes
- ASCII 14 — SO
- ASCII 15 — SI
- ASCII 16 — DLE
- ASCII 17 — DC1
- ASCII 18 — DC2
- ASCII 19 — DC3
- ASCII 20 — DC4
- ASCII 22 — SYN
- ASCII 23 — ETB
- ASCII 24 — CAN
- ASCII 25 — EM
- ASCII 26 — SUB
- ASCII 27 — ESC
- ASCII 28 — FS
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