unicode - What does it mean when my text is displayed as boxes?

Question

I'm attempting to display some text in my program using (say) Windows GDI and some of the unicode characters are displayed as boxes? What is up?

See also: What does it mean when my text is displayed as Question Marks?

Answer 1

(t-shirt image from http://www.cafepress.com/nucleartacos.163046834)

The above image represents a common failure mode of text display systems that fail to take into account the full range of Unicode characters. On Windows platforms, characters that cannot be represented using any of the available fonts are represented with an open rectangle.