avoid latex table thicker line

Question

I make a table like this one :

\documentclass[pdfa,sl,draft,english]{letter}
\usepackage[flushleft]{threeparttable}
\begin{document}
\begin{table}
\begin{center}
\begin{threeparttable}
\caption{Table caption}
\begin{tabular}{ | c | c| c | c |}
\hline
a 
& \multicolumn{1}{|p{3cm}|}{\centering b \\  (unit) } 
& c  ${}^{a}$
& d ${}^{b}$ \\ \hline
1 & 1b &  1c & 1d \\
2 & 2b &  2c & 2d \\ \hline
\end{tabular}
\begin{tablenotes}
\small
\item   ${}^{a}$ Note for c column
\item  ${}^{b}$ Note for d column
\end{tablenotes}
\end{threeparttable}
\end{center}
\end{table}
\end{document}

The compilation make a thicker right line on the first column and first row. Do you have an idea to avoid this?

Answer 1

This works. It is a bit of a hack, but essentially you set the columns to a width that you predefine. It seems to look okay like this.

\documentclass[pdfa,sl,draft,english]{letter}
\usepackage[flushleft]{threeparttable}
\usepackage{makecell}
\newcolumntype{?}{!{\vrule width 0.1pt}}

\begin{document}

Table With Bold Line
\begin{tabular}{ | c | c | c | c |}
\hline
a 
& \multicolumn{1}{|p{3cm}|}{\centering b \\  (unit) } 
& c  ${}^{a}$
& d ${}^{b}$ \\ \hline
1 & 1b &  1c & 1d \\
2 & 2b &  2c & 2d \\ \hline
\end{tabular}

Table Without Bold Line
\begin{tabular}{ ? c ? c ? c ? c ?}
\hline
a 
& \multicolumn{1}{|p{3cm}|}{\centering b \\  (unit) } 
& c  ${}^{a}$
& d ${}^{b}$ \\ \hline
1 & 1b &  1c & 1d \\
2 & 2b &  2c & 2d \\ \hline
\end{tabular}

\end{document}