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White is to play and mate in two moves. Have a go at solving this first and then, when
you are ready, click the button below to see the solution and a commentary explaining
what the problem is all about.
When solving a twomover it is a good idea to consider black moves that appear to be very
strong. One such defence in this position is 1...f2 since this gains a flight for the bK
at f3. The only efficient way of dealing with this is for White to arrange a threat of
2.Sd2#. Unfortunately this knight is pinned and if the wK moves off the b-file then he
will be checked by the bQ or by the bPd2. If we look more closely at this pawn we notice
that, by extension of the line of the bRd7, 1...d2 will be a defence against the threat.
In turn a white line of guard to e3 is opened. Similarly, after 1...Sf1 a white line of
guard to e5 is opened. We must be on the right lines here! There are two sensible
possibilities. Try 1.Rb5? (>2.Sd2) 1...d2/Sf1 2.Sg5/Sc5. In the first mate, White can
cut the line h6 to e3 because Black has opened the line c3 to e3. In the second mate,
White can cut the line from b5 to e5 because Black has opened the line from h2 to e5.
1...Qf4 is met by 2.Qxf4. However, there is no answer to 1...Rd5! since the bK can now
escape to e5. Key 1.Qb6! (>2.Sd2) 1...d2/Sf1 2.Sc5/Sg5. (By-play – 1...Rd4 2.Qxd4) The
wQ now controls e3 from a different direction so that 2.Sc5 follows the opening of the
line of wRc3. 2.Sg5 follows the opening of the line of wBh2 as wRh5 remains on the east
side of the e5 square. You will notice that the mates following 1...d2/Sf1 have been
switched around between the two phases. This strategy is known as the reciprocal change
of mates. A delightful geometry has brought the effect about. The try, 1.Rb5, is said to
be a critical move because it crosses over the critical e5 square. The key, 1.Qb6, is
said to be a pericritical move because it moves around the e3 square while continuing to
control it. The composer intends that repeated mates after different black defences will
stimulate the solver to analyse the mechanism by which this occurs.