Dougal's wife, Aife crafted a fine set of calipers from the tusks of some forgotten beast... each arm of the caliper is about as long as a troll's arm. The calipers can "measure" anything that can be placed between them: distance, weight, length, purity, honesty, love, hate, etc.
Dougal legends tell of many Unseelie lords whose plans were foiled when pinched in Aife's calipers. Some even theorize that the calipers led to the House's exile from Arcadia. A bard of House Liam sings a song of the calipers revealing some secret plot involving the High King of Arcadia.