Open line for checkmate attack (2 moves)
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This is a very common checkmate combination, that often includes sacrificing material.
Black can open the line from h5 to h1 with the knight sacrifice Nfg3+.
White is forced to capture the knight with hxg3, and Black can checkmate with Qh5#.
This combination often includes a deflection of the blocking piece, as in the following example:
White can open the line from a6 to c8 with the queen sacrifice Qxc6+.
Black is forced to capture the queen with bxc6. The pawn is also deflected from protecting a6, and White can checkmate with Ba6#. This is a common way to reach Boden's mate.
Black can open the line from e1 to g1 with the queen sacrifice Qxf2+.
If White replies Kh1, Black can checkmate with Qxf1#.
If White captures the queen with Rxf2, the rook is also deflected from protecting e1, and Black can checkmate with Re1#.
Note that the bishop on b6 is pinning the rook, preventing Rf1. This combination also includes clearance, as the queen cleared the way for the rook from e5 to e1. It is a common way to force backrank mate against the castled king, if the rook on the f-file is the only piece guarding the back rank, and there is a bishop on the diagonal.
A line can also be opened for the checkmate move (instead of for the checkmate attack).
It is also possible to open a line for a fork attack.