Twin chess problems happen by slightly altering the position to obtain a new problem that requires its own solution. The diagrammed position is regarded as part (a) while each new position formed is denoted (b), (c), etc. In general the rules for twins must be a single change to the matrix such as: shifting a piece, removing a piece, adding a piece, substituting a piece, rotating, or reflecting the board itself. To date, about 10% of my problems have included a twinning. Twinning is a very common mechanism in helpmates but less so in direct mates.
The Zagoruiko theme is named after the great Belarus composer Leonid Zagoruiko. A Zagoruiko requires at least two Black defenses to be met by different mates across at least three phases. For example, a 3x2 (3 phases) and (2 defenses) Zagoruiko could have the pattern of set, virtual, and actual play as follows:
There are thousands of 3x2 Zagoruikos in existence, but fewer 3x3s, and even less 3x4s, 4x2s, etc.
On the side I like to dabble in chess problem composition. I am mostly interested in two and three move direct mates. I hope to convey the beauty and logic of chess problems with this blog. In the entries are some of my favorite problems and my own problems. Before looking at the problems I suggest reading this introduction to the chess problem world by the British Chess Problem Society. Also, here is a list of terminology and themes. Here is a link to my problems on yacpdb.
1. "Choose wisely" The Problemist Supplement, Sept. 2016
2. "Double checking white in a two mover" StrateGems July 2016
3. "Double check without capture" The Problemist Supplement, January 2018
4. "The disappearing Nowotny: Part I" The Problemist Supplement, March 2018
5. "The disappearing Nowotny: Part II" The Problemist Supplement, May 2018
6. "The disappearing Nowotny: Part III" The Problemist Supplement, July 2018
7. "Castling with half-battery and Fleck themes" StrateGems, July 2018
8. "The Baku Nowotny" StrateGems, January 2019
9. "The Romanian Nowotny with Fleck" The Problemist, March 2019
10. "Mirror Image" The Problemist Supplement, May 2019
11. "White King in Check" Problemas, July 2019
12. "A Simple Mechanism", StrateGems, July 2019
13. "Miniatures with castling and (partial) Fleck" Problemist Supplement, September 2019
14. "Taking the Straitjacket off the Fleck" The Problemist Supplement, November 2019
15. "Unforced threats" The Problemist Supplement, May 2020
16. "Ojanen in Miniature" The Problemist Supplement, May 2020
17. "Developments in the Finnish Nowotny" The Problemist, July 2020