Black, 6/0 Thread.

A medium-weight black silk or synthetic tying thread (roughly 140 denier) suited to mounting heavy married wings and palmered hackles on full-dress Atlantic salmon flies. Stronger and slightly bulkier than 8/0 trout threads, with enough tooth to lock the wing marriage on patterns like Jock Scott, Black Doctor, and Durham Ranger. Pearsall's Naples silk in black is the traditional choice; UTC 140 or Veevus 6/0 are modern equivalents.