pin down
     v 1: define clearly; "I cannot narrow down the rules for this
          game" [syn: {peg down}, {nail down}, {narrow down}, {narrow},
           {specify}]
     2: attach with or as if with a pin; "pin up a picture" [syn: {pin
        up}]
     3: define clearly; "Can you pin down the difficulties" [syn: {narrow
        down}]
Source: WordNet® 2.0