Long, unreadable sentences in her students' essays led Rachel Foster to improve her post-16 students' punctuation. Her journey resulted, however, in more than improved punctuation. It led her to theorise what historians are really doing in their "signpost sentences". She found herself showing students how an academic historian anticipates a chunk of argument in a single, well-turned, opening sentence. Foster created an intervention in which students worked out why Richard Pipes ordered his clauses as he did. Thus they began to see Pipes' control of ideas was manifest in his control of sentence structure. Situating this within history teachers' publications on teaching writing, Foster shows how her project extends the existing field.