There are special character which are not user in conjunction with other characters. They occupy their own spacing positions.

Mnem	Mnemonic name	Unicode name	Unicode
<+->	Plus or minus	Plus-Minus sign	00B1
<AE>	AE	Latin Capital Letter Ae	00C6
<ae>	ae	Latin Small Letter Ae	00E6
<ain>	Ayn	Modifier Letter Turned Comma	02BB
<alif>	Alif	Modifier Letter Apostrophe (Was Modifier Letter Right Half Ring)	02BC (was 02BE)
<cyt>	Copyright sign	Copyright Sign	00A9
<deg>	Degree sign	Degree Sign	00B0
<eth>	Eth	Latin Small Letter Eth	00F0
<flat>	Musical flat sign	Music Flat Sign	266D
<hkO>	Hook O	Latin Capital Letter O With Horn	01A0
<hko>	Hook o	Latin Small Letter O With Horn	01A1
<hkU>	Hook U	Latin Capital Letter U With Horn	01AF
<hku>	Hook u	Latin Small Letter U With Horn	01B0
<i!>	Inverted exclamation mark	Inverted Exclamation Mark	00A1
<i?>	Inverted question mark	Inverted Question Mark	00BF
<mdot>	Middle dot	Middle Dot	00B7
<mz>	Miagkii znak	Modifier Letter Prime	02B9
<OE>	OE	Latin Capital Ligature Oe	0152
<oe>	oe	Latin Small Ligature Oe	0153
<p>	Pound	Pound Sign	00A3
<pat>	Patent	Registered Sign	00AE
<poL>	Polish L	Latin Capital Letter L With Stroke	0141
<pol>	Polish l	Latin Small Letter L With Stroke	0142
<scyt>	Sound recording copyright	Sound Recording Copyright	2117
<shrp>	Musical sharp sign	Music Sharp Sign	266F
<slO>	Slash O	Latin Capital Letter O With Stroke	00D8
<slo>	Slash o	Latin Small Letter O With Stroke	00F8
<ssl>	Script small l	Script Small L	2113
<Thn>	Thorn uppercase	Latin Capital Letter Thorn	00DE
<thn>	Thorn lowercase	Latin Small Letter Thorn	00FE
<tki>	Turkish i	Latin Small Letter Dotless I	0131
<tz>	Tverdyi znak	Modifier Letter Double Prime	02BA
<XD>	Cross D	Latin Capital Letter D With Stroke	0110
<xd>	Cross d	Latin Small Letter D With Stroke	0111
		Latin Small Letter Sharp S	00DF
		Euro Sign	20AC