Pattern # 34 This technique is explained in ebook Fundamental Chess Patterns
The fortress is an endgame drawing technique in which the side behind in material sets up a zone of protection around
their king that cannot be penetrated by the opponent. This only works when the opponent does not have a passed pawn or cannot create one,
unless that pawn can be stopped .
An elementary fortress is a theoretically drawn (i.e. a book draw) position with reduced material in which a passive defense will maintain the draw.
Fortresses usuallly have the following characteristics: Useful pawn breakthroughs are not possible. If the stronger side has pawns, they are firmly blocked. The stronger side's king cannot penetrate, either because it is cut off or near the edge of the board. Zugzwang positions cannot be forced, because the defender has waiting moves available.
Additional different patterns can be found on this page of mine:
This technique is explained in ebook Fundamental Chess Patterns
click on e7 to flip board, d8 to get the PGN
Side to move: