How do I write ð when only English letters are allowed?

Use d. That's how the machine-readable line of an Icelandic passport spells it, so Guðrún becomes Gudrun and Guðmundsson becomes Gudmundsson. Don't use th, which is the usual stand-in for þ, the other Icelandic th letter, because mixing the two up produces spellings that don't match anyone's documents.

Why does my code editor show ≡ instead of ===?

Your font draws ligatures. Coding fonts like Fira Code and JetBrains Mono render the three characters of === as one joined glyph that looks close to ≡, but the file still contains three equals signs. If it bothers you, turn ligatures off in the editor, which in VS Code is `"editor.fontLigatures": false`.

How do I make a ≡ menu button accessible?

Give the button a real label, `aria-label="Menu"`. Screen readers announce the symbol by its character name, so ≡ comes out as identical to, which tells nobody the button opens navigation. If the ≡ sits in its own span inside the button, mark that span aria-hidden="true" so it isn't read at all.

How do I check a ≡ b (mod n) in code?

Test whether the difference divides evenly: `(a - b) % n === 0`. Comparing a % n with b % n breaks in JavaScript, C and Java for negative numbers, because % there keeps the sign of the left operand, so -1 % 5 is -1 and never equals 4 % 5. Python's % returns a non-negative result for positive n, so both forms work there.

ASCII 240

Lowercase ethIdentical to (triple bar)

ASCII 240 is ð on Windows and ≡ in old DOS.

ð (Small Eth) is byte 240 in Windows-1252, Unicode U+00F0. Icelandic and Faroese write it for the th in words like fjörður, and phonetics uses it for the th sound of this. As a byte, F0 matters for emoji. Emoji above U+FFFF, which covers the faces, hands and animals, take four bytes in UTF-8 starting with F0, so emoji read as Windows-1252 begin with ð: 😂 becomes ðŸ˜‚, and a page sprinkled with ðŸ is full of broken emoji. The same byte explains a classic MySQL error. The old utf8 charset stores at most three bytes per character, so inserting an emoji fails with Incorrect string value: '\xF0\x9F\x98\x82'. Switch the column and the connection to utf8mb4. The UTF-8 bytes of ð itself are C3 B0, which misread as Ã°. The HTML entity is ð.

≡ (Identical To) is byte 240 in code page 437, the original IBM PC character set, and maps to Unicode U+2261. Maths uses it for identities and congruences, as in 17 ≡ 2 (mod 5), and chemistry for a triple bond, HC≡CH. Sites also borrow it as a menu icon, alongside the trigram ☰ (U+2630). Code page 850 put the soft hyphen on this byte, so a ≡ from a 437 file read as 850 doesn't turn into another symbol, it vanishes, because the soft hyphen is invisible. In UTF-8, ≡ is E2 89 A1, and the HTML entity is &equiv;.

Latin LetterMath Symbol

Technical Details

Code Example

</>

#include <stdio.h>

int main(void) {
    /* On Windows (code page 1252) byte 240 is ð.
       Modern terminals expect UTF-8, so print the code point, not the byte. */
    printf("\u00F0\n");   /* UTF-8: C3 B0 */

    unsigned char b = 240;         /* the raw Windows-1252 byte */
    printf("%d 0x%02X\n", b, b);    /* 240 0xF0 */
    return 0;
}

Code Example

</>

#include <stdio.h>

int main(void) {
    /* Byte 240 is ≡ only on a console using code page 437
       (chcp 437 on Windows, or DOSBox). On a UTF-8 terminal the
       lone byte 0xF0 is invalid and prints as garbage, often �. */
    putchar(240);

    /* char is signed on x86, so a plain char holding 0xF0 is -16.
       Use unsigned char when you compare or index by byte value. */
    char c = (char)240;
    unsigned char u = 240;
    printf("\n%d %d\n", c, u);   /* -16 240 */

    /* Portable: print the Unicode character as UTF-8 instead. */
    printf("\u2261\n");          /* E2 89 A1 */
    return 0;
}

How to Type It

Windows: Hold Alt and type 0240 on the numeric keypad, with Num Lock on. The leading zero picks the Windows code page. No numeric keypad? In Word, type 00F0 and press Alt+X.
Mac: Press Control+Command+Space to open the Character Viewer, then search for "latin small letter eth".
Linux: In GNOME, or any desktop running IBus, press Ctrl+Shift+U, type f0, then Space. With a Compose key set up, press Compose, d, h.
Phone: Add the Icelandic keyboard in your phone's keyboard settings and type ð there, or tap the character at the top of this page to copy it.

How to Type It

Windows: Hold Alt and type 240 on the numeric keypad, with no leading zero. US PCs insert ≡, but PCs set to code page 850, common in Western Europe, insert a soft hyphen. No numeric keypad? In Word, type 2261 and press Alt+X.
Mac: Press Control+Command+Space to open the Character Viewer, then search for "identical to".
Linux: In GNOME, or any desktop running IBus, press Ctrl+Shift+U, type 2261, then Space. With a Compose key set up, press Compose, equals, underscore.
Phone: Tap the character at the top of this page to copy it.

Frequently Asked Questions

Use d. That's how the machine-readable line of an Icelandic passport spells it, so Guðrún becomes Gudrun and Guðmundsson becomes Gudmundsson. Don't use th, which is the usual stand-in for þ, the other Icelandic th letter, because mixing the two up produces spellings that don't match anyone's documents.

Frequently Asked Questions

Your font draws ligatures. Coding fonts like Fira Code and JetBrains Mono render the three characters of === as one joined glyph that looks close to ≡, but the file still contains three equals signs. If it bothers you, turn ligatures off in the editor, which in VS Code is `"editor.fontLigatures": false`.
Give the button a real label, `aria-label="Menu"`. Screen readers announce the symbol by its character name, so ≡ comes out as identical to, which tells nobody the button opens navigation. If the ≡ sits in its own span inside the button, mark that span aria-hidden="true" so it isn't read at all.
Test whether the difference divides evenly: `(a - b) % n === 0`. Comparing a % n with b % n breaks in JavaScript, C and Java for negative numbers, because % there keeps the sign of the left operand, so -1 % 5 is -1 and never equals 4 % 5. Python's % returns a non-negative result for positive n, so both forms work there.

ASCII 240

Lowercase ethIdentical to (triple bar)

ASCII 240 is ð on Windows and ≡ in old DOS.

Latin Letter

Math Symbol

#include <stdio.h>

int main(void) {

/* On Windows (code page 1252) byte 240 is ð.

Modern terminals expect UTF-8, so print the code point, not the byte. */

printf("\u00F0\n"); /* UTF-8: C3 B0 */

unsigned char b = 240; /* the raw Windows-1252 byte */

printf("%d 0x%02X\n", b, b); /* 240 0xF0 */

return 0;

}

#include <stdio.h>

int main(void) {

/* Byte 240 is ≡ only on a console using code page 437

(chcp 437 on Windows, or DOSBox). On a UTF-8 terminal the

lone byte 0xF0 is invalid and prints as garbage, often �. */

putchar(240);

/* char is signed on x86, so a plain char holding 0xF0 is -16.

Use unsigned char when you compare or index by byte value. */

char c = (char)240;

unsigned char u = 240;

printf("\n%d %d\n", c, u); /* -16 240 */

/* Portable: print the Unicode character as UTF-8 instead. */

printf("\u2261\n"); /* E2 89 A1 */

return 0;

}