The chess problems, orthodox or heterodox, contain various themes (that is certain arrangements of the pieces, or certain series of moves), which add beauty and interest to their solutions. The themes are presented in a number of moves (themes of two moves, themes of three moves) or in a number of phases.
Initial phase, set play : We suppose (without moving any pieces) that the first move, the key, has already been played. The continuation of the game, starting from this position, (defenses of the black in an orthodox problem, mates of the white), is called set play. If there exists “set play” in a problem, its stipulation will contain a star (*).
Not all problems have set play.
Intermediate phases, virtual play : A trial move T is played in order to solve the problem but the opponent finds a unique defense D and stops the attack. We call a try the move T. We write these moves with question mark and exclamation mark: 1 T? D!
The continuation of the game, starting with the try, is called virtual play. A problem may have many tries, that is many intermediate phases. (Intermediate phases are also created with the mechanism of production of twin problems). The tries make the discovery of the key more difficult, thus the tries make the problem more interesting.
Not all problems have tries.
Final phase, actual play : We find the move K, the key that solves the problem. The continuation of the game, starting with the key, is called actual play. The key is unique, except when the composer specifies more solutions. We write the key-move with an exclamation mark : 1 Κ!
Problems without a key are considered useless, because of “no solution”.
According to the number of phases we have two-phase problem, three-phase problem, four-phase problem, etc. Some problems have too many (let us say 25) phases.
In every phase there are one or more variations, different series of black and white moves. The variations of one phase may be related with the variations of another phase. The variations which are related with the theme are called thematic variations. The other variations are called by-play. The by-play is sometimes interesting.
[This Post in Greek language]