1. The criminal must be introduced early on, and must not be a
     character whose thoughts the reader has followed.

  2. No supernatural agencies.

  3. No more than one secret room or passage, and these should only be
     found in appropriate buildings.

  4. No undiscovered poisons, or appliances which need a long
     scientific explanation at the end.

  5. No racist caricatures.

  6. The detective must not be helped by an accident or unaccountable
     intuition.

  7. The detective must not commit the crime.

  8. The detective must not find any clues which are not immediately
     revealed to the reader.

  9. 'The Watson', i.e. the detective's friend who is often the main
     point of view character, must be of slightly lower intelligence
     than the average reader, and must not conceal any thoughts which
     go through his mind.

  10. Twins and doubles must not be introduced unless the reader has
      been prepared for them.The 14 Features of Eternal Fascism