original_query: What is the purpose of the sner (or Fleißner) grille in an 8x8 grid?
query: What is the purpose of the Fleissner grille in an 8x8 grid?
answer: The Fleissner grille in an 8x8 grid makes 16 perforations (4 in each quadrant).  It has four positions (North, East, South, West), each exposing 16 of the 64 squares. The encipherer writes 16 letters of the message in each position, rotating the grille 90 degrees after each set of 16 letters.