The RS Agreement is mentioned in paragraph 16.3 of the Code. But in such a context that I want to cry.

... then the Retro-Strategy (RS) convention should be applied: whichever castling is executed first is deemed to be permissible.

I'm not talking about the case of dependence of castling between different parties (here an agreement may be just for clarity), although in fact no agreement is needed here, it is enough to apply paragraph 16.1 twice (in the initial position and after the first castling).

But regarding the interdependence of castling, which ONE SIDE has, it sounds just wild!

...... then the Retro-Strategy (RS) convention should be applied: whichever castling is executed first is deemed to be permissible.

It sounds beautiful, but in fact it's nonsense!

I hesitate to ask, but if our castling would not be interdependent, and we would have the right to both, then what? Would we do two? Really? Or anyway, in this case, "the first one would be considered acceptable"? :)

Attention, question! Why describe the obvious things that we can do using the usual rules 16.1 and 16.2 and call them the beautiful word Agreement? The question is actually rhetorical.

And the following is not a rhetorical question. Are there any tasks that require the application of the RS agreement because the standard clauses of rules 16.1 and 16.2 are not enough to solve?

It would be extremely interesting to look at such a task. I've never met anyone like that before.


Extract from the Codex:

Article 16 – Castling and En-passant capture

(1) Castling convention. Castling is permitted unless it can be proved that it is not permissible.

(2) En-passant convention. An en-passant capture on the first move is permitted only if it can be proved that the last move was the double step of the pawn which is to be captured.

(3) Partial Retrograde Analysis (PRA) convention. Where the rights to castle and/or to capture en-passant are mutually dependent, the solution consists of several mutually exclusive parts. All possible combinations of move rights, taking into account the castling convention and the en-passant convention, form these mutually exclusive parts. If in the case of mutual dependency of castling rights a solution is not possible according to the PRA convention, then the Retro-Strategy (RS) convention should be applied: whichever castling is executed first is deemed to be permissible.

  • 2
    This question would be easier to understand with more context, and spelling out abbreviations like RS and Codex. Commented Jul 8 at 7:32

2 Answers 2



Retro Strategy & Partial Retro Analysis are "protocols". They describe how to apply the two base conventions for optimistic (e.g. castling) & pessimistic (e.g. ep) conditional moves. They do not themselves permit or prevent specific moves.

The difference between RS & PRA is the most important thing:


  1. Retro analysis.
  2. Play a permitted move.
  3. Determine what combos of conditional moves remain valid.
  4. Go to 2.


  1. Retro analysis.
  2. For each dominant combo of conditional moves, create a part.
  3. Solve each part in turn.


I completely agree that Article 16 of the Codex is badly written. You are an intelligent & diligent person, and are far from the only one who is still confused or disappointed after reading Codex Article 16, even when also having read Werner Keym's excellent tutorial.

The Codex does not have the same holy status as the FIDE Laws of Chess. We do not have to take it literally. As it says in the Codex Introduction:

It is not intended to be a body of established law which problemists must observe on pain of being condemned of heresy or worse; problemists are independent spirits, and it would be pointless for the WFCC to attempt to legislate in that way.

But retro is all about figuring out the consequences of rules & conventions. So isn't it a blocker if the conventions are unclear. The answer is to let go of the literal wording of the Codex (even your favourite Art 16.2). The Codex is just a sketch.

Instead, we have thousands of retro problems composed in the past. We can look to them to understand the points the composers were making through the chessboard. Most of them were conforming to a relatively simple set of principles. Others deliberately flouted those principles. And the principles also evolved over time.

There is definitely a more coherent set of conventions that can be written. However, it would be a mistake to try to construct a rigid model into which everything must fit. There will always be exceptions, and the rules & conventions will always evolve.

The final point is that most retros these days don't involve the conventions. Proof games and pure retros do not care about forward play. Instead, these conventions apply only to so-called "retroactive" problems which combine both forward and backward play. In the old days, almost all retros were retroactive, and although it's rather out of fashion today, in my opinion there is a lot remaining to be done creatively in this space, particularly if we are able to gently make it more rigorous.


Before answering the question, you first need to define the terms. What is the protocol, what is the method, what is the Convention?

Initially, we have two basic Conventions - on castling and on en-passant (16.1 and 16.2). Both Conventions relate only to direct play and do not allow making retroanalytical conclusions (for example, the position of the black king on e8 and the black rook on h8) is not a basis for asserting that black DEFINITELY has the right to castling and vice versa (the position of the white pawn on b6 and the black pawn on c6) is not a basis for conclusion: "If we can't prove that the last move was c7-c5, then the last move was DEFINITELY different". The last move is still possible and c7-c5, but we cannot prove it.

Protocol for solving ANY problem:

1) Retroanalysis (we determine the presence / absence of all possible special rights: castling, e.p.). An optimistic right (castling) is checked for the possibility; a pessimistic one (en-passant) for the possibility of proving it.

2) We make the allowed move.

3) We check the situation with the rights, taking into account the progress made (now only the optimistic rights to the opportunity - the pessimistic has long been gone).

4) Repeat step 2. Make the allowed move (etc. indefinitely, alternating points 2 and 3).

Any task solved in this way DOES NOT REQUIRE additional Conventions. This protocol cannot even be called RS (which happens all the time and where the confusion comes from). It's just a STANDARD PROTOCOL for solving ANY problem.


But the imagination of the authors, as you know, is boundless. And they come up with tasks that cannot be solved by the usual method and for which an additional Convention is required.

The most common tasks of this kind are partial type problems, where the solution must be divided into parts, and then solve each part separately. I will not consider these tasks in my answer at all because they are not relevant to my question at all. A convention for such tasks was required until 2008 only because the method of division into parts itself was not legal. After 2008, some of the tasks that came under the PRA Convention became legal by default, but it still does not matter directly to the question of RS. Only indirect (due to the similarity of the methods used in solving the problem).


What is the RS method? What is its essence?

In a nutshell: the elimination of those possible realities in the past THAT DO NOT allow us to solve the problem.

Almost no one ever pays attention to what I have highlighted (I sin this myself) - therefore, cases that have nothing to do with it are constantly attributed to RS (like Keym's recent task, for example).


The classic case of using the RS Method is the case of the interdependence of white and black castling. To solve the problem, we NEED (it is necessary, it is important!) that black does not have the right to castling. The only way to prove it is to make your castling.

Standard Protocol:

  1. Retroanalysis (showed the possibility of castling by both white and black. Despite the fact that they are interdependent, any party can make them in its turn, according to 16.1).
  2. We make a legal move (castling, according to 16.1).
  3. The analysis shows that black's castling is impossible (according to 16.1). Hooray! We can solve the problem.

Notice! We didn't use ANY CONVENTION. We used ONLY the STANDARD PROTOCOL.


What is the conclusion of all this? It's very simple!

RS (Retro Strategies) is neither a Convention nor even a protocol. It's just a Method!

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