[Tex/LaTex] Eliminate space after section heading

sectioningspacing

I have set the spacing after subsubsection to 1sp, but there is still an appreciable space between the heading and the text (approx 1ex). Obviously, I can't set a negative value, and I don't want to manually create a negative space at the start of every subsubsection.

Any suggestions on how to do this? Is there a length variable that I should be setting?

\documentclass[10pt,letterpaper]{article}
\usepackage[letterpaper,margin=0.75in]{geometry}
\usepackage[utf8]{inputenc}
\usepackage{mdwlist}
\usepackage[T1]{fontenc}
\usepackage{textcomp}
\usepackage{tgpagella}
\pagestyle{empty}


\begin{document}
\setlength{\parskip}{0em}
\setlength{\parindent}{0em}

\makeatletter
\renewcommand\paragraph{\@startsection{paragraph}{4}{\z@}%
                                    {1.25ex \@plus1ex \@minus.2ex}%
                                    {-0em}%
                                    {\normalfont\normalsize\bfseries}}
\renewcommand\subsubsection{\@startsection {subsubsection}{3}{\z@ }%
                                           {-1.5ex\@plus -1ex \@minus -.2ex}%
                                           {1sp \@minus 1ex}%
                                           {\normalfont \normalsize \bfseries }}
\makeatother
\subsubsection*{Blah}
This text is 1ex from the heading blah, but I would like it to be hard up 
against the heading (but not on the same line)
\end{document}

I need to be able to have some headings which have some space, and some which have none, so I can't use a package which simply affects the whole document.

Best Answer

I am not sure if this is what you want (or why do you want it), but you can use \nointerlineskip and then annihilate the effect of \lineskip:

\documentclass[10pt,letterpaper]{article}
\usepackage[letterpaper,margin=0.75in]{geometry}
\usepackage[utf8]{inputenc}
\usepackage{mdwlist}
\usepackage[T1]{fontenc}
\usepackage{textcomp}
\usepackage{tgpagella}
\pagestyle{empty}

\begin{document}
\setlength{\parskip}{0em}
\setlength{\parindent}{0em}

\makeatletter
\renewcommand\paragraph{\@startsection{paragraph}{4}{\z@}%
  {1.25ex \@plus1ex \@minus.2ex}%
  {-0em}%
  {\normalfont\normalsize\bfseries}}
\renewcommand\subsubsection{\@startsection {subsubsection}{3}{\z@ }%
  {-1.5ex\@plus -1ex \@minus -.2ex}%
  {1sp \@minus 0ex\nointerlineskip\vspace{-\lineskip}}%
  {\normalfont \normalsize \bfseries }}
\makeatother

\subsubsection*{Blah}
This text is 1ex from the heading blah, but I would like it to be hard up 
against the heading (but not on the same line)

\end{document}

An some text of the resulting document zoomed at 400% to see the resulting heading and following text:

enter image description here

On a side note, setting \parskip and \parindent to 0pt might not be a good idea (the l2tabu document contains some details).

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