Please consider this file:
\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{center}
\begin{tikzpicture}[scale=6]
\draw (0,0)--(1,0)--(1,1)--(0,1)--cycle;
\end{tikzpicture}%
\begin{tikzpicture}[scale=6]
\draw (0,0)--(1,0)--(1,1)--(0,1)--cycle;
\end{tikzpicture}\par
\begin{tikzpicture}[scale=6]
\draw (0,0)--(1,0)--(1,1)--(0,1)--cycle;
\end{tikzpicture}%
\begin{tikzpicture}[scale=6]
\draw (0,0)--(1,0)--(1,1)--(0,1)--cycle;
\end{tikzpicture}
\end{center}
\end{document}
Obviously, the output should consist of four squares of the same size, two above and two below (plus the page number). What I got can be seen here:
As you can see, there is no horizontal space between the two top squares, and there is also no space between the two bottom squares. However, there is a small vertical space between the first pair and the second one. Why? And how can I remove it without trial and error. I know that adding \vspace{-1pt} at the right spot (that is, right after \par) solves my problem, but, precisely, that amount of space was obtained through trial and error.

\lineskip=0ptin the center environment. – Ulrike Fischer Dec 29 '23 at 19:18