Hello, This is from the nvidia matrix class : matrix4 & mult_right( const matrix4 & b ) { matrix4 mt(*this); set_value(real(0)); for(int i=0; i < 4; i++) for(int j=0; j < 4; j++) for(int c=0; c < 4; c++) element(i,j) += mt(i,c) * b(c,j); return *this; } matrix4 & operator *= ( const matrix4 & mat ) { mult_right( mat ); return *this; } And it's used like this : matrix4f m_matrix = P * M; It's not a standard matrix * matrix operation, what are they doing here ? thanks, Bruno