Hello List, Continuing from the last post.. I have the following defined: struct element_t { element_t( unsigned char value, unsigned char x, unsigned char y ):value(value),x(x),y(y){} unsigned char value; unsigned char x; unsigned char y; }; typedef boost::multi_index_container< element_t, boost::multi_index::indexed_by< /*value*/ boost::multi_index::ordered_non_unique< boost::multi_index::member< element_t, unsigned char, &element_t::value > >, /*x co-ordinate*/ boost::multi_index::ordered_non_unique< boost::multi_index::member< element_t, unsigned char, &element_t::x > >, /*y co-ordinate*/ boost::multi_index::ordered_non_unique< boost::multi_index::member< element_t, unsigned char, &element_t::y > >, /*unique values per x co-ordinate*/ boost::multi_index::ordered_non_unique< boost::multi_index::composite_key< element_t, boost::multi_index::member< element_t, unsigned char, &element_t::x >, boost::multi_index::member< element_t, unsigned char, &element_t::value > > > > > matrix_t; How would I define the "unique values per x co-ordinate" as there x co-ordinate is suppose to be non unique while value in this key should ONLY point to unique instances within that x co-ordinate? Many Thanks, Etienne