[Un]Common Checkmate Patterns: Bishop + Knight [Part 3 of 3]

This is actually the simpler of the 2 phases:  once you achieve "the position" [shown below], there is a forced M19, which isn't as tough as it sounds as it involves 3 repeating themes.

The White King is on f6, the Knight on f7, and the Bishop on h7.

Black has several options but none allow him to escape.  Beware of following the engine evaluation because it can't see the forced mate and is constantly leading the viewer astray.  Just follow the below line, master it, and then worry later about engine evals.

Note that we're in the opposite corner where checkmate will occur [corner square is dark and the Bishop's square is light].

Note also that the Bishop controls the light squares and the Knight, because it's also on a light square, controls the dark squares.  This complementary action is critical.

And the King, because it has opposition, protects the Knight and prevents the enemy King from moving diagonally.

We want to keep the enemy King on the back rank if at all possible.  He will temporarily escape but our mating net will haul him back in.

Trying to herd the King is like trying to herd cats:  possible but difficult and demanding a lot of patience.

My diagrams will have colors representing control by a piece:

• Yellow=Bishop
• Green=Knight
• Blue=King

Here is the starting position:

White has just played 1. Bh7, cutting off Black's retreat into the corner.

Obviously, the enemy King has only one square to move.  [One] Mission accomplished.

Now the enemy King will make a break for it:  try to get off the back rank and escape to open territory.  Where does he want to go?  e8 then d7:

White could prevent that with Ke6 but then Black could just retreat to f8 and nothing is gained.  Instead, we block with 2. Ne5 Kd8:

3. Ke6 Kc7  4. Nd7 Kb7 and now it looks as if Black will escape but White has the answer:  5. Bd3.

Notice the barrier that the Bishop and Knight present.  This will be a recurring theme.

Black moves 5. ... Kc6 but notice he has no way out:

The mistake here is to make a Bishop check, which will ruin the barrier.  Instead, make a waiting move and force Black to retreat [Zugzwang]:

6. Be2 [White maintains control of the a6-f1 diagonal but don't retreat all of the way to f1 for reasons we'll see later] Kc7  7. Bf3, preventing the King from getting back to c6.

This was the toughest part of the puzzle after "the position".

7. ... Kc8  8. Kd6 Kd8  9. Bh5 [this is why we didn't play 6. Bf1:  because then we couldn't have gotten to h5].

Notice how similar this position is to the starting position:  if the Bishop were on f7, they'd be identical.  We've taken the entire initial position and shifted it over 2 squares closer to the checkmate corner.

Note that even if Black had played 7. ... Kd8, his escape to f8 would have been blocked by the Knight.

Now we're in the home stretch.

9. ... Kc8 [forced]  10. Nc5, cutting off b7 [and analogous to 2. e5] Kd8  11. Nb7+:

11. ... Kc8  12. Kc6 [maintaining opposition, protecting the Knight] Kb8  13. Kb6 [same comment as before] Kc8  14. Bg4+:

And now the coup d'grace:  14. ... Kb8  15. Nc5 [moving AWAY from the corner; not Na5 as we'll see shortly] Ka8  16. Bd7 [another waiting move, staying away from c8 and the reach of the enemy King; White absolutely does NOT want to play 16. Na6 as that is stalemate!] Kb8  17. Na6+ Ka8  18. Bc6#.

There!  No problem, right?

OK, maybe a slight problem of learning the ideas and executing the plan.  But practice it and you'll eventually get it.  Set up the starting position in the engine [see my blog post on how to set up positions in the engine] and hit the "Practice vs Computer" icon and try to replicate the above lesson, knowing that Stockfish could choose different options but the end result will be the same.

https://www.chess.com/blog/EnPassantFork/how-to-use-the-engine-to-analyze-any-position

Notice the attacker's King stayed on the 6th rank the entire time.

Themes:

• Maintaining the barrier:  very few checks:  mostly restricting the enemy King's movement
• Zugzwang:  recognizing that the enemy must retreat if it were his move
• Opposition:  keeping the King directly opposed to the enemy King as often as possible to prevent him from gaining more space