The keys defined on a X11 system running can be looked up in following file:
/usr/include/X11/keysymdef.h
To find out if the specified file above is on your running system use following command:
locate keysymdef.h
Searching for special key:
user@host~ grep -i space /usr/include/X11/keysymdef.h
#define XK_BackSpace 0xff08 /* Back space, back char */
#define XK_KP_Space 0xff80 /* Space */
#define XK_ISO_Partial_Space_Left 0xfe25
#define XK_ISO_Partial_Space_Right 0xfe26
#define XK_space 0x0020 /* U+0020 SPACE */
#define XK_nobreakspace 0x00a0 /* U+00A0 NO-BREAK SPACE */
#define XK_emspace 0x0aa1 /* U+2003 EM SPACE */
#define XK_enspace 0x0aa2 /* U+2002 EN SPACE */
#define XK_em3space 0x0aa3 /* U+2004 THREE-PER-EM SPACE */
#define XK_em4space 0x0aa4 /* U+2005 FOUR-PER-EM SPACE */
#define XK_digitspace 0x0aa5 /* U+2007 FIGURE SPACE */
#define XK_punctspace 0x0aa6 /* U+2008 PUNCTUATION SPACE */
#define XK_thinspace 0x0aa7 /* U+2009 THIN SPACE */
#define XK_hairspace 0x0aa8 /* U+200A HAIR SPACE */
Anoter useful X tool to find out a key name is called xev. This tool is part of the X.org server pacage.