math spaces List of predefined spaces: Math Spaces  Command  Example  Result  \, (smallest predefined)  $$a\,b$$   \: (second smallest predefined)  $$a\:b$$   \; (third smallest predefined)  $$a\;b$$   \/ (avoiding ligatures)  $$V\/A$$ instead of $$VA$$  instead of  \quad (space of current character set size)  $$a\quad~b$$   \qquad (double space of current character set size)  $$a\qquad~b$$   \_ (where _ is blank!)  $$a\ b$$ (whereas $$a\b$$ is not a valid filter expression since the blank space is missing; it is recommended to use the tilde ~ instead of the simple whitespace) 
 \hspace{n} ,where n positive integer (= n Pixels)  $$a~\hspace{30}~b$$ $$a~\hspace{15}~b$$ $$a~\hspace{2}~b$$ $$a~\hspace{1}~b$$ 
 \unitlength{m}\hspace{n}, changes the default unit length (m=1px) to be applied  $$a~\hspace{2}~b\unitlength{10}~\hspace{2}~c$$ (second space is 10x2=20px)  

Note: Simple blank spaces and tildes (~) are ignored by the TeX filter and don't produce any space. You must use one of the defined math spaces to get a visible (extra) space. 