'a red thread'

There is a curious custom in the British Navy: all the cordage of the Royal Fleet, whether heavy or light, is twisted so that a red thread runs through whole ropes, by which even the smallest piece can be recognized as Crown property.

In the same way a thread of love and deep attachment seems to run through Ottilie's diary, connecting everything she writes, and giving it a distinctive character. Because of this thread, the young girl's comments, observations and quotations bear a special mark, and convey a particular meaning. Each passage we have selected and recorded is definite proof of this.

—Goethe, Elective Affinities (1809) (trans. Elizabeth Mayer & Louise Bogan 1963)