The Square with Upper Left To Lower Right Fill symbol (▧). Copy and paste it, or use its Unicode, HTML, and CSS codes.
More from Geometric Shapes.
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+25A7 (decimal 9639).
Use ▧ in HTML to render ▧ on a web page.
On Windows you can type it with Alt+9639 (hold Alt and enter the number on the numeric keypad).
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