# Negative \vspace* too small

\documentclass{article}

\begin{document}
\vspace*{-3em}
\rule{3em}{3em}
\end{document}


Why does the rule protrude into the text area?

As a comparison, this is how it looks without the \vspace:

And this is what I would have expected:

You have to consider both \topskip and \lineskip.

\documentclass{article}