When the display for some characters does not work, you will see little empty boxes instead of the glyphs you would like to see.

One way to debug this is to check whether you have the fonts necessary for this character set. If you do, then you will have to check wether your fontset has the correct mapping from charset to font (see FontSets).

Check character set

Put point on the empty box, and use `C-u C-x =’. The result should tell you the charset you want to know. Here is an example from the HELLO file (use ‘C-h h’ to display it):

   character: ? (07067, 3639, 0xe37)
     charset: cyrillic-iso8859-5
	      (Right-Hand Part of Latin/Cyrillic Alphabet (ISO/IEC 8859-5): ISO-IR-144)
  code point: 55
      syntax: word
    category: y:Cyrillic  
 buffer code: 0x8C 0xB7
   file code: ESC 2C 4C 37 (encoded by coding system iso-2022-7bit-unix)
	font: -- none --

When I go to check the list of installed fonts, I find none for ISO 8859-5:

    ~% xlsfonts | grep 8859-5

I do, however, have fonts for other encodings, such as 8859-1:

    ~% xlsfonts | grep 8859-1 | head -n 3
    -adobe-avant garde gothic-book-o-normal--0-0-0-0-p-0-iso8859-1
    -adobe-avant garde gothic-book-r-normal--0-0-0-0-p-0-iso8859-1
    -adobe-avant garde gothic-demi-o-normal--0-0-0-0-p-0-iso8859-1

That explains why my Emacs cannot display a character in ISO 8859-5. There just is no font capable of it. [Note that Emacs can do arbitrary mapping of characters onto font glyphs via ‘font-ccl-encoder-alist’, so the above isn’t the whole story; see below. For instance, you might display Cyrillic with fonts encoded in any of iso8859-5, koi8-r, windows-1251 or iso10646-1, though vanilla Emacs 21.2 only supports the first two. – fx]

Check font set

Call M-x describe-fontset to see what the current fontset defines:

    Fontset: -*-*-medium-r-normal--14-130-75-75-c-70-fontset-startup
    ---------------------	---------
    ascii			-misc-fixed-medium-r-normal--14-130-75-75-c-70-iso8859-1
    latin-iso8859-1		-misc-fixed-*-iso8859-1
    latin-iso8859-2		-*-iso8859-2
    latin-iso8859-3		-*-iso8859-3
    latin-iso8859-4		-*-iso8859-4
    cyrillic-iso8859-5	-*-*-*-iso8859-5
    latin-iso8859-9		-*-iso8859-9
    latin-iso8859-15	-*-iso8859-15

What this means is that Emacs will use a font matching -*-*-*-iso8859-5 for any character of the charset cyrillic-iso8859-5. And as we have seen above, there is none.

If you believe you have the correct fonts installed, this is the place to check whether Emacs will consider them.