1. Dashes separate

      (a dash is longer than a hyphen — to make one type shift-option-minus, or just use two hyphens -- but whichever you choose, stick to it).
      1. Always put a space before and after a dash.

      2. Use dashes sparingly — most people use them far too often.

      3. Use a dash to indicate a pause or break in the sentence.

        The prisoners sat in Poetry Appreciation Chairs strapped in. Vogons suffered no illusions as to the regard their works were generally held in.

        Whatever Zaphod's qualities of mind might include
        dash, bravado, conceit he was mechanically inept and could easily blow the ship up with an extravagant gesture.
      4. Use a dash to add an after- or before-thought to a complete sentence.

        A red star the size of a small plate crept across it followed quickly by another one a binary system.

        Old, ugly, and stupid
        yes, he had it all.
      5. Use a dash to indicate an interruption or cut-off, especially in dialog (where it goes inside the quotation marks).

        He thought again, which required looking at the ceiling, "Some of the shouting I quite like." He filled his lungs and bellowed, "Resistance is —"

        "Sure, yes," interrupted Ford hurriedly, "you're good at that, I can tell."