The mark is copy-desk shorthand for "to come," and like much of that trade's private orthography — hed, dek, graf, lede — it is deliberately misspelled so that it can never be mistaken for the thing itself. The digraph tk is close to orthographically impossible in English, which makes it trivially searchable: a page can be swept for unfinished business before it goes to press without a single false positive. Magazine shops, Time Inc.'s in particular, ran on it. Hed TK. Art TK. Fact-check TK. Doubled to TKTK when the hole was larger than a phrase, or stretched across a whole paragraph the writer knew had to exist but could not yet write. What survives in drafting software today is that habit, formalized: a placeholder that announces its own incompleteness, holds its place in the argument, and waits to be replaced by the sentence it was standing in for.