Drawn as separated marks rather than one line. In this house's boards, a dashed edge means the format is not yet in the lexicon — an honest unknown, not an absence.
Middle English daschen (to strike violently, to dart); drawn as separated marks rather than one unbroken line, each mark a small strike rather than a continuous stroke.
Each dash is a small strike rather than a continuous stroke, the old sense of daschen still visible in the broken line. An edge drawn this way marks an honest gap in the lexicon, not a missing thing.