# How to align a right angle

As you you can see in the following image, a right angle has some space to side BC.

How can I fix it and is there any better way to add an right angle? \documentclass{article}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
\draw[fill=gray!10]  (0, 4) coordinate (A)
-- node[above=4pt] {$4cm$} (3,4) coordinate (C)
-- node[right] {$8cm$} (3,0) coordinate (B)
-- node[left] {$xcm$}  (0, 4);
\draw (2.6,4) -- ++(0,-10pt) -- ++(10pt,0);

\node at (A)[anchor=east] {$A$};
\node at (B)[anchor=north] {$B$};
\node at (C)[anchor=south] {$C$};
\end{tikzpicture}
\end{document}


Since the little square has side 10pt, you need to start 10pt to the left of C using, for example

([xshift=-10pt]C)


A complete example:

\documentclass{article}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
\draw[fill=gray!10]  (0, 4) coordinate (A)
-- node[above=4pt] {$4$\, cm} (3,4) coordinate (C)
-- node[right] {$8$\,cm} (3,0) coordinate (B)
-- node[left] {$x$\,cm}  (0, 4);
\draw ([xshift=-10pt]C) -- ++(0,-10pt) -- ++(10pt,0);

\node at (A)[anchor=east] {$A$};
\node at (B)[anchor=north] {$B$};
\node at (C)[anchor=south] {$C$};
\end{tikzpicture}
\end{document} If you are going to do a lot of constructions like this one, I'd like to suggest you the tkz-euclide package; it has a very intuitive syntax and has a predefined command \tkzMarkRightAngle to easily mark the right angles:

\documentclass{article}
\usepackage{tkz-euclide}
\usetkzobj{all}

\begin{document}

\begin{tikzpicture}
\tkzDefPoint(0,4){A}
\tkzDefPoint(3,4){C}
\tkzDefPoint(3,0){B}
\tkzDrawPolygon[fill=gray!10](A,B,C)
\tkzLabelPoints(B)
\tkzLabelPoints[left](A)
\tkzLabelPoints[right](C)
\tkzMarkRightAngle(B,C,A)
\tkzLabelSegment[above](A,C){$4$\,cm}
\tkzLabelSegment[left=4pt](B,A){$x$\,cm}
\tkzLabelSegment[right](C,B){$8$\,cm}
\end{tikzpicture}

\end{document} You can also use the right angle symbol style: ## Code:

\documentclass{article}
\usepackage{tikz}
\usepackage{siunitx}
\usetikzlibrary{calc}

\tikzset{
right angle quadrant=1, % Make sure it is set, even if not called explicitly
right angle length/.code={\def\rightanglelength{#1}},   % Length of symbol
right angle length=2ex, % Make sure it is set...
right angle symbol/.style n args={3}{
insert path={
let \p0 = ($(#1)!(#3)!(#2)$) in     % Intersection
let \p1 = ($(\p0)!\quadranta*\rightanglelength!(#3)$), % Point on base line
\p2 = ($(\p0)!\quadrantb*\rightanglelength!(#2)$) in % Point on perpendicular line
let \p3 = ($(\p1)+(\p2)-(\p0)$) in  % Corner point of symbol
(\p1) -- (\p3) -- (\p2)
}
}
}

\begin{document}
\begin{tikzpicture}
\draw[fill=gray!10]  (0, 4) coordinate (A)
-- node[above=4pt] {$\SI{4}{\cm}$} (3,4) coordinate (C)
-- node[right] {$\SI{8}{\cm}$} (3,0) coordinate (B)
-- node[left] {$x$ \si{\cm}}  (0, 4);
%\draw (2.6,4) -- ++(0,-10pt) -- ++(10pt,0);

\draw [red,ultra thick, right angle quadrant=2,right angle symbol={B}{C}{A}];

\node at (A)[anchor=east] {$A$};
\node at (B)[anchor=north] {$B$};
\node at (C)[anchor=south] {$C$};
\end{tikzpicture}
\end{document}