I always wondered what a was called.

Unicode input

Glyph availability

[…]

If an application does not have access to a glyph, the character will usually be shown as the font’s .notdef glyph ⟨􏿮⟩ which often appears as an empty box, ☐ (nicknamed “tofu” based on the shape), a box with an X in it, ☒, a diamond with a question mark, �, or a box with a question mark in it, ⍰.

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