The Dingbat Negative Circled Sans-Serif Digit Six symbol (➏). Copy and paste it, or use its Unicode, HTML, and CSS codes.
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The quickest way is to copy ➏ from this page and paste it anywhere. You can also insert it with its Unicode, HTML, or alt code listed above.
➏ is U+278F (decimal 10127).
Use ➏ in HTML to render ➏ on a web page.
On Windows you can type it with Alt+10127 (hold Alt and enter the number on the numeric keypad).
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