Large negative spaces

In text mode: \kern-1em equals "minus quad"

In math mode: \mkern-18mu equals "minus quad"

For "minus qquad" use -2em or -36mu respectively.

For LaTeX, horizontal spacing is achieved using \hspace{<len>} where <len> is a length (either positive or negative); TeX's equivalent to \hskip. Since \quad [\qquad] is equivalent to a horizontal skip of 1em [2em], use \hspace{-1em} [\hspace{-2em}] to obtain a negative space amount.

\kern inserts a space depending on the mode that TeX is in, and could therefore be either vertical or horizontal.

Text-based spacing can be inserted using any of the \phantom-related commands: \phantom{<stuff>}, or \hphantom{<stuff>}, for example.