multipart shape horizontal split

Question

How come the horizontal split seems not to work whereas the vertical one does ?

\documentclass[12pt,standalone]{article}
\usepackage{tikz}

\begin{document}
\usetikzlibrary {shapes.multipart}
 \begin{tikzpicture}[my shape/.style={rectangle split horizontal , rectangle split parts=#1, draw}]
  \node [my shape=2] 
  {a  \nodepart{two} b };
 \end{tikzpicture}
 
 \begin{tikzpicture}[my shape/.style={ rectangle split , rectangle split parts=#1, draw}]
  \node [my shape=2] 
  {a \nodepart{two} b };
 \end{tikzpicture}  
\end{document}

Answer 1

You are missing rectangle split in your horizontal node:

\documentclass[12pt]{article}
\usepackage{tikz}

\begin{document}
\usetikzlibrary {shapes.multipart}
 \begin{tikzpicture}[my shape/.style={
  rectangle split, 
  rectangle split horizontal , 
  rectangle split parts=#1, 
  draw
  }]
  \node [my shape=2] 
  {a  \nodepart{two} b };
 \end{tikzpicture}
 
 \begin{tikzpicture}[my shape/.style={ rectangle split , rectangle split parts=#1, draw}]
  \node [my shape=2] 
  {a \nodepart{two} b };
 \end{tikzpicture}  
\end{document}