176. J. Willis Sydney Mail 1879 

Mate in 2 
The surprising key 1.Qd7! (waiting) grants the black king access to e4. Taking the flight results in a triple pinmate: 1…Kxe4 2.Qxe6. When Black moves one of the three pieces that were pinned in the first variation, a line is opened and White regains control of e4: 1…R~ 2.Qd6, 1…Sd~ 2.Sf3, and 1…Sf~ 2.Sd3. Lastly 1…B~ enables 2.Qxd4, guarding the flight directly.
Dennis Hale: The key had me bamboozled for much longer than I normally take. The variation 1…Kxe4 2.Qxe6 is first class. Black's move selfpins the three defenders of e6. Bravo J. Willis! Your idea still furrows the brow as it would have some 135 years ago.
177. C. D. Fethers The Australasian Chess Review 1931 

Mate in 2 
The strong defence 1…Sxf5 without a set mate provides a hint for the key, 1.Qd3! (threat: 2.Qe3), which answers 1…Sxf5 with 2.Qxf5. The wellused white queen gives four other mates: 1…bxc5 2.Qa6 (a switchback), 1…Rxe2+ 2.Qxe2, 1…Sc4 2.Qxd5, and 1…d4 2.Qe4. The white knight gets unpinned twice with 1…Bd2 2.Sd4 and 1…Bf2 2.Sf4. And including 1…Sf7 2.Bxf7 the problem delivers a total of eight variations.
Dennis Hale: Most pleasing are the two pairs of variations, (1) 1…bxc5 2.Qa6 and 1…Rxe2+ 2.Qxe2; (2) 1…Bd2 2.Sd4 and 1…Bf2 2.Sf4. My heart sings when I am interacting with a wellconstructed twomover!
178. Thomas Henderson 777 Chess Miniatures 1908 

Mate in 3 
The eyecatching diagram has just six pieces that are nicely spread out on the board. The waiting key 1.Rf7! preserves the set play for 1…Kd6 with 2.Bd4 Kd5 3.Rd7, while providing for 1…Ke5 with 2.Bc5 Kd5 3.Rf5. Both variations end with model mates, in which the five pieces form first a straight line, and then a cross.
Dennis Hale: Two pretty, closelyrelated variations.
179. F. W. Walton Chess World 1950 

Mate in 2 
Except for 1…Qxh1, every black move has been given a set mate, e.g. 1…f4 2.Qxe4, and 1…Qf3 2.Qxf3. The excellent key 1.Rg2 (waiting) unpins the black queen, and its ‘random’ move 1…Q~ allows 2.Rgd2. Four correction moves by the queen disable 2.Rgd2 but permit new white responses: 1…Qxc2+ 2.Rxc2, 1…Qe2 2.Rxe2, 1…Qf3 2.Bxf3 (a changed mate), and 1…Qxg2 2.Qxg2. Another change from the set play is 1…f4 2.Rg5, repinning the queen. Two more variations, 1…S~ 2.Rgd2 and 1…d6 2.Sxb6, round off a good traditional work.
Dennis Hale: I like the unpinning key, and my favourite variation is 1…f4 2.Rg5.
180. Denis Saunders StrateGems 1999 

Mate in 2 
After 1.e4! to cut off the black queen, White threatens 2.Sf5. Three captures of the keypawn are answered by different queen mates: 1…dxe3 e.p. 2.Qe5, 1…fxe3 e.p. 2.Qh2, and 1…Qxe4 2.Qa3. The en passant captures produce some curious line effects; the vanishing white pawn means that the black queen regains control of f5, but a black pawn’s arrival on e3 interferes with the rook on the efile and the queen on the third rank. A fourth queen mate occurs with 1…Qh3 2.Qxd4, while one further queen defence 1…Qxb5 enables 2.Bxb5. Two variations involve unpinning the e6pawn: 1…Sf6 2.Bc7 and 1…Rxg6 (or 1…Rf7) 2.Sf7. Lastly, the selfinterference 1…Se3 allows 2.e5.
Dennis Hale: The key is excellent, and the three longrange queen mating moves (after the captures of the e4pawn) are a feature of the problem.
181. Charles G. M. Watson Chess 1949 

Mate in 2 
Set play is provided for all black moves, including 1…S~ 2.Qg7 and 1…d3 2.Qa1. White has no simple waiting move available, however, e.g. 1.Kb2? d3!, 1.Ka3? cxb4+, or 1.Kb3? c4+! The key 1.Qf2! (waiting) changes the black knight variations to 1…S~ 2.Qxh4 and 1…Sxf4 2.Qxf4, while a concurrent change occurs with 1…d3 2.Qb2 – so this problem is a mutate. The remaining play is as set: 1…Bf7 2.Bg7, 1…Bg6 2.Sxd7, 1…Bh~ 2.Bg5, 1…c4/cxb4 2.Qxd4, and 1…cxd5 2.Sxd5.
Dennis Hale: I particularly like the pair of selfblocking variations, 1…Bf7 2.Bg7 and 1…Bg6 2.Sxd7.
182. Frederick Hawes & Frank Ravenscroft Themes64 1957 

Mate in 3 
The key 1.Bg7! covers h6 and h8, so as to free the white knights from guarding these squares. The only threat is 2.f7 though, aiming for 3.Sf6. Black has three moves that prepare to defend f6, but they allow the g6knight to be activated in different ways. 1…Rb3 permits 2.Kf7 followed by 3.Sf8, since 2…Ba2 no longer checks. If 1…c2 (intending 2…Bc3), the b1bishop is cut off and White answers with 2.S6e7; now 2…f4 is ineffective against 3.Bg6 (here the g6knight must choose e7 to disable 2…Bb4+). Finally, 1…Sf2 interferes with the f1rook and White continues with 2.Sf4 and 3.Bg6, as 2…Rxf4 is ruled out. Three refined variations based on black selfinterference, with another connecting feature: all white second moves (including the threat) are motivated by squarevacation.
183. Andy Sag OzProblems.com 24 May 2014 

Mate in 2 
White starts with an active sacrifice of the queen, 1.Qg4! (waiting), which completes the block by guarding f5 and obstructing the g5pawn. Three black pawn captures enable a variety of mates: 1…hxg4 2.Sxg4, 1…gxh4 2.Qxg6, and 1…gxf5 2.Qxf5. The selfblock 1…Bg7 permits 2.Be7, while the remaining defences are unguards: 1…Sh~ 2.Qxg5, 1…Sb~ 2.Sd7, and 1…c6/c5 2.Rd6. The subtle try 1.Rf4? (waiting) produces similar play – with these differences: 1…gxf5 2.Rxf5, 1…h4 2.Sg4, and 1…gxf4 2.Qxg6 – but it is refuted by 1…g4!
Dennis Hale: I like the sacrificial key. The try 1.Rd7? (threatening 2.Rf7) is defeated by 1…Sxd7!
Nigel Nettheim: The d2rook and c7pawn take part in a minor battle; they have been added to avoid the cook 1.Sg4+ and 2.Qb2. The cook could otherwise have been avoided simply by adding a white pawn in one of several possible positions such as f2, although the rookandpawn method has the advantage of adding a variation.
Nigel Nettheim: The d2rook and c7pawn take part in a minor battle; they have been added to avoid the cook 1.Sg4+ and 2.Qb2. The cook could otherwise have been avoided simply by adding a white pawn in one of several possible positions such as f2, although the rookandpawn method has the advantage of adding a variation.
184. Frederick Bennett The Brisbane Courier 1928 

Mate in 2 
After the flightgiving key 1.Sb3!, White threatens the battery mate 2.Rxc5. If Black takes the flight, White fires the battery again with 1…Kxe2 2.Re4. All moves by the d5knight defend by opening a line for the g8bishop, and a random placement 1…S5~ allows 2.Qe4. Three correction moves by the knight disable 2.Qe4 but provoke new mates: 1…S5c3 2.Rd4, 1…S5e3 2.Rd2, and 1…Sf6 2.Sxf4. The d1knight has three defences, though two result in the same play as that created by the first knight: 1…S1c3 2.Rd4, 1…S1e3 2.Rd2, and 1…Sb2 2.Sc1. This last variation, in which a black knight interferes with a linepiece and enables a knight mate, nicely matches 1…Sf6 2.Sxf4.
Nigel Nettheim: The play is excellent, especially 1…Sb2 2.Sc1 which distinguishes the key from the try 1.Sf3?
185. Anthony Vickers Chess in Australia 1971 

Mate in 2 
The thematic key 1.Sxf5! selfpins the knight and threatens 2.Qe4. Black’s h5knight sets off two variations involving sophisticated line play and dual avoidance. 1…Sf6 unpins the keypiece, which apparently has a choice of two mating moves. But the black defence has also cut off the h6rook, and if 2.Se7? – interfering with the e8rook – then the king would escape to e6, meaning only 2.Se3 works. 1…Sf4 again unpins the white knight, but now the h4queen is obstructed and White must avoid 2.Se3, since that would interfere with the g1bishop and permit 2…Kxd4; so 2.Se7 is forced. The byplay consists of two pairs of loosely related variations: 1…Sc3/Sd6 (unguard) 2.Rd6, 1…Sxd4 (square clearance) 2.Qxd4, 1…Qg4 (unguard) 2.Bxf7, and 1…Qxe5 (square clearance) 2.Rxe5.
Nigel Nettheim: The f5pawn is a strong defender and is eliminated, even at the cost of selfpinning. 1...Sh~ leads nicely to two different mates with the keypiece, which has regained its freedom. The c6bishop, rather than a c6pawn, prevents the cook 1.Sxb5.
186. Andras Toth Australasian Chess 2013 

Helpmate in 2½ Twin (b) Swap Re4 and Kf5 
The diagram is solved by 1…Rxg4 2.Qg6 Re4 3.g4 Rf4, and part (b) by 1…Rxd5 2.Bd3 Rf5 3.d5 Re5. In both solutions, the white rook captures a pawn and performs a switchback. The aim of the each capture is to clear the square for another black pawn, which must move to unguard the mating square. The white rook then mates by swinging to the opposite direction once more. Good analogy between the two phases (including the black selfblocks). Note how the black queen is used in part (b) to prevent the alternative move 2…Rxg5 from working.
Nigel Nettheim: Very enjoyable to solve.
187. E. D. McQueen Daily News 1927 

Mate in 2 
Set mates have been prepared for all black moves in the diagram: 1…Sc~ 2.Se7 and 1…Sd~ 2.Sf6. But White has no waiting move that could preserve these variations. A good key 1.Sf7! (waiting) gives the black king a flight, which leads to 1…Kxe6 2.Bc4, a model mate. The set play is completely replaced by 1…Sc~ 2.Rd6 and 1…Sd~ 2.Re5. This lovely mutate was sent to me by the UK expert Michael McDowell, who found it while researching in the library of the British Chess Problem Society.
Nigel Nettheim: A fine sacrifice and a complete mutate. Economical, too: the b6pawn neatly prevents a simple waitingmove key with the b5bishop moving on its long diagonal.
Dennis Hale: I like the light setting coupled with the sacrificial key. A pretty little problem.
Dennis Hale: I like the light setting coupled with the sacrificial key. A pretty little problem.
188. Cyril Whitehead Chess in Australia 1986 

Mate in 2 
The key 1.d4! prepares to fire the R + S battery with 2.Sd3 (which also opens the indirect B + S battery to cover the flight on g5). In response to various black defences, the f4knight delivers seven more mates with each of its possible moves, to produce a very fine knighttour. 1…Qxd5 2.Sxd5, 1…Sxe6 2.Sxe6, 1…Bxg6 2.Sxg6, 1…Sxh5 2.Sxh5, 1…Rxh3+ 2.Sxh3, 1…Rg/Rhxg2 2.Sxg2, and 1…Sxe2 2.Sxe2. The flighttaking 1…Kg5 doesn’t disable the threat 2.Sd3, but with the king mated on a different square, this may be regarded as a distinct variation that makes the knighttour even more complete! There’s one secondary line, 1…f5 2.Qd8.
Dennis Hale: This is a beautifully constructed problem. The 1…f5 2.Qd8 variation makes clear why the try 1.e7? (threat: 2.Se6) is defeated by 1…f5! – delightful.
Nigel Nettheim: The f4horse (the one with the Whitehead) is chafing at the bit to discover mate, muzzled by its own pawns. The position is legal, although that is not obvious. Six captures were made with white pawns, accounting for a black bishop and five pawns. It turns out that Black could make the required promotions, assisted by the capture of a white rook.
Nigel Nettheim: The f4horse (the one with the Whitehead) is chafing at the bit to discover mate, muzzled by its own pawns. The position is legal, although that is not obvious. Six captures were made with white pawns, accounting for a black bishop and five pawns. It turns out that Black could make the required promotions, assisted by the capture of a white rook.
189. Mark Wilson Chess in Australia 1987 

Mate in 3 
The key 1.Bxc7! seems to be defeated by 1…00, but here we can prove that castling is illegal by using retroanalysis, where we deduce the play that must have occurred prior to the diagram position. The white bishop on d8 could not have reached that square by normal means because of the black pawns on c7 and e7, which have never moved. The bishop is therefore a promoted pawn that arrived on d8 from d7. When on d7 the pawn attacked e8, meaning the black king must have moved previously to answer or avoid the check. So Black, having moved the king before, cannot castle in response to the key. The threat is 2.Rd8 mate, and if 1…Kf8 then 2.Rd8+ Kg7 3.Be5.
Nigel Nettheim: A neat and simple example of retroanalysis in a directmate.
Dennis Hale: A nice retro idea – technically, the d8bishop is termed an obtrusive piece.
Dennis Hale: A nice retro idea – technically, the d8bishop is termed an obtrusive piece.
190. Arthur Charlick The Sydney Morning Herald 1903 1st Prize 

Mate in 2 
The key 1.Qh4! entails an unobvious threat, 2.Qe1. White has placed the queen in an ambush position behind the black rook, and if 1…Rg4 (or 1…Rg3, etc.) then 2.Qxe7. 1…Rf5 disables both the threat and 2.Qxe7 by cutting off the f7rook’s control of f4, but allows 2.Rxe7 due to selfblock. 1…Sf5 blocks the same square, enabling the white bishop to interfere with the f7rook: 2.Bf6. The analogous selfblock 1…Sd5 permits the white knight to interfere with the d2rook: 2.Sd3. 1…d5 defends by creating a flight on d6, and is answered by 2.Bxc7. Also, 1…Bd4 2.Qxd4 and 1…Sh~ 2.Qf4.
Dennis Hale: Very difficult to solve. The white queen leaps into a nest of black men, seemingly disadvantageously restricting her mobility. Particularly pleasing are the variations 1…Sf5 2.Bf6, 1…Sd5 2.Sd3, and 1…d5 2.Bxc7. A delightful problem.
191. Geoff Foster Chess in Australia 1985 

Mate in 2 
A prominent set variation, 1…Rc3+ 2.Qxc3, is abandoned by the key 1.Qb8!, which creates a battery on the bfile and threatens 2.B5any. Now 1…Rc3+ provokes a new mate, 2.Bd3. The two white rooks can be captured in four different ways, generating two pairs of dualfree variations in spite of the multiple threats: 1…Kxa5 2.Bxc4, 1…Kxc5 2.Bxa4, 1…Rxa5 2.Rxc4, and 1…Rxc5 2.Rxa4. So the two black rooks are captured in four different ways as well!
Dennis Hale: The key is not easy to see as it removes the set counter to 1…Rc3+ (2.Qxc3) and replaces it with 2.Bd3. The problem is a comprehensive exercise in dual avoidance.
Nigel Nettheim: Extremely neat, with four sacrifices, marvellous symmetry, and a check by Black with a changed mate. (It does not matter at all that there are several threats.) Bravo!
Nigel Nettheim: Extremely neat, with four sacrifices, marvellous symmetry, and a check by Black with a changed mate. (It does not matter at all that there are several threats.) Bravo!
192. William Whyatt Problem 1960 

Mate in 3 
Black has only two possible moves, and both are provided with set play: 1…Sxc2 2.Qb2 S~ 3.Qxh2 and 1…Sxb3 2.Qb2 S~ 3.Qb8. If 1.Kxh2? then 1…Sxc2!, or if 1.b4? then 1…Sb3! The key 1.Qb2! creates another block position. Now after 1…Sxc2 White has only one available waiting move 2.b4, forcing 2…S~ 3.Qxh2. Similarly after 1…Sxb3 White can maintain the block only with 2.Kxh2, leading to 2…S~ 3.Qb8. Technically a mutate, though the changes between the set and actual play are not as dramatic as that seen in many Whyatt problems.
Nigel Nettheim: The tries 1.Qd2? (2.Qxh2, 2.Qe3) 1...Sf5! and 1.Qb4? (2.Qb8, 2.Qc5) 1...Sxe6! show that the battle between the queen and the d4knight is won by the knight. Instead of impatiently threatening either of those mates on the b8h2 diagonal, White must more subtly step back with 1.Qb2!, keeping both those mates in mind for the future. So the battle between the queen and the a1knight is won by the queen. The restraint shown in giving up an immediate attack and instead stepping back to lie in wait is the most appealing feature for me.
Dennis Hale: A delicate waiter with the queen mating at one end of the b8h2 diagonal in the first variation, and the other end of that diagonal in the second. A very fine problem.
Dennis Hale: A delicate waiter with the queen mating at one end of the b8h2 diagonal in the first variation, and the other end of that diagonal in the second. A very fine problem.
193. M. D. Harpur Check! 1944 

Mate in 2 
The excellent key 1.Qf2! concedes two flights to the black king and threatens 2.Sd4. When the king accesses these flights, White uses the f4knight to execute two doublechecks, discovered from different directions: 1…Ke5 2.Sg6 and 1…Kf6 2.Sd5. The Q + S battery, initially controlled by the a2rook and a8bishop, fires again after the b4knight interferes with one of these linepieces, allowing White to shut off the other one: 1…Sc6 2.Se2 and 1…Sc2 2.Sd5. Lastly, 1…Rd5 2.Se2 (somewhat distractingly similar to one of the main variations) and 1…Bg7 2.Sxg7.
Nigel Nettheim: The key 1.Qf2! is not hard to find. The variations are good, especially the flight ones. The a2rook stops 1.Se2. The h3pawn is not needed for soundness, but it stops the toostrong defence 1...Rxh2+ which would almost give away the key.
Dennis Hale: The problem is a real eyeopener in lineopenings and lineclosures.
Dennis Hale: The problem is a real eyeopener in lineopenings and lineclosures.
194. Laimons Mangalis Chess Life 1959 

Mate in 2 
Two important set variations are 1…c4 2.Qa5 and 1…e4 2.Qxg5. After the key 1.Sxd4!, which threatens 2.Sxc2, the two pawn defences yield new mates: 1…c4 2.Ra5 and 1…e4 2.Rf5. Further, the two queen mates seen in the set play are transferred to another pair of defences: 1…cxd4 2.Qa5 and 1…exd4 2.Qxg5. Such a combination of changed play and mate transference with respect to two thematic defences is named the Rukhlis theme. Three black knight variations make up the byplay: 1…Sxd4/Sb4/Se1 2.Se3, 1…Se6/Sf3 2.e4, and 1…Se4 2.Bf7.
195. F. W. Walton The Observer 1946 

Mate in 3 
In this miniature, the waiting key 1.Bf3! leads to various attractive mating configurations. 1…Kf5 2.Se7+ generates two model mates with 2…Ke6 3.Bd5 and 2…Kg5 3.Qc1. The distant selfblock 1…Bg5 permits 2.Qe7+ Kf5 3.Qe4, while other bishop moves unguard f6: 1…B~ 2.Qe7+ Kf5 3.Qxf6 – another model mate. Lastly, 1…f5 is answered by 2.Kc4, threatening 3.Bd5, and if 2…f4 then 3.Bg4. The latter variation ends with a mirror mate, in which the mated king is surrounded by eight empty squares. Thanks to Michael McDowell who discovered this “forgotten gem” while researching in the library of the British Chess Problem Society.
196. Linden Lyons Probleemblad 2011 

Mate in 2 
After the key 1.Qb5!, White threatens 2.Qd3. All moves by the d5knight defend by creating a potential flight, and a random placement 1…Sd~ is answered by 2.Qf5. The knight has three specific moves that prevent the secondary threat of 2.Qf5, but they commit new errors that enable White to mate in other ways: 1…Se7 2.Qe5 (selfinterference), 1…Sf6 2.Sd6 (another selfinterference), and 1…Se3 2.f3 (selfblock). One minor variation is provided by the other black knight: 1…Sb4/Sc5 2.Sc5.
Nigel Nettheim: 1...Sd~ allows 2.Qf5, except for three “corrections” 1...Se7, 1...Sf6 and 1...Se3. The first two each shut off one of the black rooks, while the third is handled in a contrasting way. The set play is retained after the key. The neat responses to the moves of the d5knight are the best feature.
Dennis Hale: One small step for a queen, one giant step for the white army. I particularly like the variation 1…Se3 2.f3.
Dennis Hale: One small step for a queen, one giant step for the white army. I particularly like the variation 1…Se3 2.f3.
197. Adrian Berkel The Brisbane Courier 1918 

Mate in 2 
Initially, almost every possible black move has been set with a mating reply, the exception being 1…dxc6. The waiting key 1.Rxb6! covers c6 so that 1…dxc6 is met by 2.Bxc6. Besides completing the block position, the fine key also adds two new variations: 1…cxb6+ 2.c7 (crosscheck) and 1…c4/cxd4 2.Rb5 (switchback). Five remaining variations are unchanged from the set play: 1…dxe6 2.Rd8, 1…d6 2.Se3, 1…b3 2.c4, 1…S~ 2.Sf4, and 1…Ba7 2.Sxc7.
Nigel Nettheim: White provides for 1…dxc6 with the surprising 1.Rxb6! The following capture with check (1...cxb6+ 2.c7) makes this an excellent key. The giving up of an en passant capture by 1...b3 2.c4 provides an appealing variation.
Dennis Hale: Arguably the best variation is 1…cxb6+ 2.c7; Black's battery is successfully assaulted by White's battery. I also like the three defences involving Black's d7pawn. An enjoyable problem.
Dennis Hale: Arguably the best variation is 1…cxb6+ 2.c7; Black's battery is successfully assaulted by White's battery. I also like the three defences involving Black's d7pawn. An enjoyable problem.
198. Arthur Mosely L’Italia Scacchistica 1918 

Mate in 2 
Two important set variations are 1…Sf6+ 2.Sgxf6 and 1…d3 2.Se3. The key 1.Qc8!, by putting an extra guard on c4, threatens 2.Rxb5. Since the queen has abandoned control of e5, the set play no longer works, but now that c5 is attacked by the same piece, the play changes to 1…Sf6+ Sexf6 and 1…d3 2.Sc3. There’s only one other variation – 1…Sb~ 2.Qd7 – in this straightforward illustration of free change, i.e. a problem with changed mates but which is not a complete block.
Nigel Nettheim: The key was a bit hard to find, perhaps precisely because it is not specially brilliant! It overprotects c4 (thus threatening 2.Rxb5) and c5, but leaves e5 under protected, changing the set 1...Sf6+ 2.Sgxf6 to 2.Sexf6 and 1…d3 2.Se3 to 2.Sc3.
Dennis Hale: Particularly pleasing are the variations, 1…d3 2.Sc3 and 1…Sf6+ 2.Sexf6.
Dennis Hale: Particularly pleasing are the variations, 1…d3 2.Sc3 and 1…Sf6+ 2.Sexf6.
199. Joseph Heydon Good Companions 1920 

Mate in 2 
One set variation, 1…d1(Q) 2.Bg2, is discarded by the key, 1.Qf4! (waiting). The black king has two flights, and accessing them gives 1…Kf1 2.Qxf2 and 1…Kd1 2.Qxd2/Qxg4. If Black moves a pawn, White exploits the resulting selfblock in different ways depending on Black’s choice of promotion piece: 1…f1(Q) 2.Qxd2, 1…f1(S) 2.Qxg4, 1…d1(Q) 2.Qxf2, and 1…d1(S) 2.Bd3. Three nice pairs of variations, in which Black plays repeatedly to d1 and f1. The thematic try 1.Qd4? (waiting) produces similar play, and involves one changed mate – 1…d1(S) 2.Qd3, but it is defeated by 1…f1(S)!
Nigel Nettheim: Giving up discovered check makes the key surprising. 1.Qd4? was tempting, but it is answered by 1...f1(S)! The scheme is very neat. The white king is positioned to prevent 1.Qb5+ Kd1 2.Qf1, and at the same time allows 1...d1(S) 2.Bd3. Similarly, the g4pawn prevents 1.Qh5+ Kf1 2.Qd1 and at the same time avoids a dual with 1...f1(S) 2.Qf3. The duals after bishop promotions are of little consequence.
200. Peter Wong U.S. Problem Bulletin 1988 

White retracts 1 move, and then mates in 2 
If White retracts a nondescript move, such as 1.Kd5e6, there is no mateintwo, since the subsequent 1.Ke6 (threat: 2.a8(Q)) is answered by 1…00! Instead, White must retract in such a way that the resulting position is one in which it can be shown – by retroanalysis – that Black cannot castle.
Dennis Hale: A witty problem. White will not be deterred!