The Lower Right Shadowed White Square symbol (❑). Copy and paste it, or use its Unicode, HTML, and CSS codes.
More from Dingbats.
The quickest way is to copy ❑ from this page and paste it anywhere. You can also insert it with its Unicode, HTML, or alt code listed above.
❑ is U+2751 (decimal 10065).
Use ❑ in HTML to render ❑ on a web page.
On Windows you can type it with Alt+10065 (hold Alt and enter the number on the numeric keypad).
Yes — copying is completely free and unlimited, and works right in your browser with no sign-up.