Command key symbol
It has no Windows Alt code: copy it from this page, or use its HTML entity or Unicode number.
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About the command key symbol
The looped square on Apple’s Command key exists because Steve Jobs felt the early Macintosh menus showed too many Apple logos. The designer Susan Kare searched a dictionary of symbols for a replacement and settled on this one, which Mac keyboards have carried since 1984.
In Scandinavia the same sign points out sights worth visiting on maps and signposts, which is why Unicode calls it the place of interest sign.