Dear devs, It is currently not possible to have nested mathmatrices.  The reason is that \popmacro\math_matrix_NC is done at an inner scope and a succeeding \endgroup will happily overwrite this again.  A possible fix is to move \popmacro\math_matrix_NC to the very outer scope in \math_matrix_stop, see below.  I don't know and have not tested whether this breaks something else. Cheers, Henri --- \unprotect \def\math_matrix_stop   {\crcr  % \ifgridsnapping \else      \mathstrut\crcr      \noalign{\vskip-\baselineskip}%  % \fi    \egroup   %\popmacro\math_matrix_NC % Too early!    \egroup    \mathmatrixright    \endgroup    \popmacro\math_matrix_NC} % ungrouped! \protect \definemathmatrix[pmatrix][left={\left(}, right={\right)}, distance=2em] \def\matOne{\startmatrix\NC 1 \NC 0 \NR\NC 0 \NC 1\NR\stopmatrix} \def\matTwo{\startmatrix\NC a\NR\NC b\NR\stopmatrix} \def\matThree{\startmatrix\NC c \NC d\NR\stopmatrix} \starttext \startformula   \startpmatrix     \NC \matOne \NC \matTwo\NR     \NC \matThree \NC e\NR   \stoppmatrix \stopformula \startformula   \startpmatrix     \NC 1 \NC 0 \NC a \NR     \NC 0 \NC 1 \NC b \NR     \NC c \NC d \NC e\NR   \stoppmatrix \stopformula \stoptext