Here's a version that looks ok. I added a \twoof command for the second brace and made a longer \phold command \Phold. Since the underlying commands are tabular you can pass any tabular specification to it so I removed the intercolumn space to make the alignment look a bit nicer.

\documentclass{article} \usepackage{phonrule} \usepackage{amsmath} \newcommand*{\twoof}[2][c]{% \ensuremath{% \left\{ \begin{tabular}{#1#1}#2\end{tabular} \right\} }% } \newcommand{\Phold}{\rule[-4pt]{2.5em}{.5pt}} \begin{document} \phonc {\phonfeat{$-$cnt\\ +hi}} {[$\alpha$grv]} {\twoof[c@{}]{% \phonfeat{$-$cnt\\ $\alpha$grv} & \Phold\\ \Phold & \phonfeat{$-$cnt\\ $\alpha$vce}% }} \end{document} 

Here's a version that looks ok. I added a \twoof command for the second brace and made a longer \phold command \Phold. Since the underlying commands are tabular you can pass any tabular specification to it so I removed the intercolumn space to make the alignment look a bit nicer.

\documentclass{article} \usepackage{phonrule} \usepackage{amsmath} \newcommand*{\twoof}[2][c]{% \ensuremath{% \left\{ \begin{tabular}{#1#1}#2\end{tabular} \right\} }% } \newcommand*{\Phold}{\rule[-4pt]{2.5em}{.5pt}} \begin{document} \phonc {\phonfeat{$-$cnt\\ +hi}} {[$\alpha$grv]} {\twoof[@{}c@{}]{% \phonfeat{$-$cnt\\ $\alpha$grv} & \Phold\\ \Phold & \phonfeat{$-$cnt\\ $\alpha$vce}% }} \end{document}