TACK ON is a term used to describe the action of adding something to an existing object or structure. The crossword solutions for this term are ADD (3 letters), APPEND (6 letters), ANNEX (5 letters), AFFIX (5 letters), ATTACH (6 letters), and APPENDTO (8 letters). All of these words refer to the same action of attaching something to something else, but they each have slightly different connotations. ADD implies a simple addition, APPEND suggests a continuation, ANNEX implies a permanent attachment, AFFIX implies a physical attachment, ATTACH implies a connection, and APPENDTO implies a specific attachment.