Quick answer
Windmill is a tactical idea: two aligned attackers on the king file. Work from the first evidence position for Windmill, and begin by testing Kg7.
Begin from Kg7 in 5k2/4p3/8/8/2Q5/8/4R3/6K1 b - - 5 3. Start with the forcing geometry: checks, captures, threats, defenders and flight squares around the target. The evidence answer is: Windmill: preserve the alignment and calculate the defensive interposition before trading the front piece. Candidate line: Kg7.
In the Windmill example, notice: side Black; pieces Black king f8, Black pawn e7, White queen c4, White rook e2, White king g1; legal first moves Kg7, Ke8, e6, e5. Anchor your reading to the FEN above; the inventory is illustrative, not a portable law of chess.
The tactical trigger
The first evidence board tests two aligned attackers on the king file; the second board tests two aligned attackers on the king file. Use direct FEN setup to reconstruct A and direct FEN setup to reconstruct B when the evidence includes move text. Shared language is catalogued on the Chess Wiki and the Tactics hub. Then compare Candidate Moves with Battery, Smothered Mate, Corridor Mate around the mating net's boundary.
The two inventories make the distinction concrete. A contains side Black; pieces Black king f8, Black pawn e7, White queen c4, White rook e2, White king g1; legal first moves Kg7, Ke8, e6, e5; B contains side White; pieces Black king e8, Black pawn e7, White rook a6, White queen f2, White king g1; legal first moves Ra7, Ra8+, Rb6, Rc6, Rd6, Re6, Rf6, Rg6. Compare the side to move before comparing the labels in the context of the exchange that removes a guard.
Topic dossier for this page and board pair r1:5k2 | r2:4p3 | r3:8 | r4:8 | r5:2Q5 | r6:8 | r7:4R3 | r8:6K1 / r1:4k3 | r2:4p3 | r3:R7 | r4:8 | r5:8 | r6:8 | r7:5Q2 | r8:6K1: start with the mating net's boundary; then test the exchange that removes a guard; keep a counter-check that changes the calculation separate from a deflection from the critical square; use the final simplification as the transfer check; and reserve the last safe flight square for the exception.
Calculation order
On the first board, calculate Kg7 after Kg7 and write down the opponent's best reply around the mating net's boundary. On the second board, calculate Ra7 after Ra7 and check the exchange that removes a guard. Calculate the most forcing move first, then test the opponent's best capture, interposition or counter-check before naming the motif. In this case the useful condition is the last safe flight square.
Key claim: Windmill is illustrated here by two aligned attackers on the king file; the first concrete test is Kg7. The board anchor is 5k2/4p3/8/8/2Q5/8/4R3/6K1; the example is intentionally narrower than the topic label and should not be generalised without checking the surrounding position..
Practical boundary: To answer “How can a player recognize windmill, calculate the winning idea, and test the defender's best resource?”, compare the recorded setup with Ra7 and the opponent's reply. The decision is useful only when the stated condition survives that concrete test on board 4k3/4p3/R7/8/8/8/5Q2/6K1. The evidence records Illustrative evidence for the stated mechanism; it is not a claim that the candidate move is forced or best in every position and the second board signature r1:4k3 | r2:4p3 | r3:R7 | r4:8 | r5:8 | r6:8 | r7:5Q2 | r8:6K1. The false-positive test is whether a counter-check that changes the calculation appears without a deflection from the critical square, so the conclusion is useful only when the stated condition survives that concrete test.
Evidence positions
example-1: two aligned attackers on the king file
Show answer
Windmill: preserve the alignment and calculate the defensive interposition before trading the front piece. Candidate line: Kg7.
Why this position is here: Windmill is represented by two aligned attackers on the king file. This is a direct FEN setup with board anchor 5k2/4p3/8/8/2Q5/8/4R3/6K1; no opening history is inferred from the diagram.
FEN: 5k2/4p3/8/8/2Q5/8/4R3/6K1 b - - 5 3
Question: What is the first concrete decision in this windmill position?
Answer cue: Windmill: preserve the alignment and calculate the defensive interposition before trading the front piece. Candidate line: Kg7. The first candidate to test is Kg7. Replay Kg7 on board anchor 5k2/4p3/8/8/2Q5/8/4R3/6K1 and write down what changes after it.
Evidence boundary: Illustrative evidence for the stated mechanism; it is not a claim that the candidate move is forced or best in every position.
example-2: two aligned attackers on the king file
Show answer
Windmill: preserve the alignment and calculate the defensive interposition before trading the front piece. Candidate line: Ra7.
Why this position is here: Windmill is represented by two aligned attackers on the king file. This is a direct FEN setup with board anchor 4k3/4p3/R7/8/8/8/5Q2/6K1; no opening history is inferred from the diagram.
FEN: 4k3/4p3/R7/8/8/8/5Q2/6K1 w - - 4 3
Question: What is the first concrete decision in this windmill position?
Answer cue: Windmill: preserve the alignment and calculate the defensive interposition before trading the front piece. Candidate line: Ra7. The first candidate to test is Ra7. Replay Ra7 on board anchor 4k3/4p3/R7/8/8/8/5Q2/6K1 and write down what changes after it.
Evidence boundary: Illustrative evidence for the stated mechanism; it is not a claim that the candidate move is forced or best in every position.
example-3: two aligned attackers on the king file
Show answer
Windmill: preserve the alignment and calculate the defensive interposition before trading the front piece. Candidate line: Re4.
Why this position is here: Windmill is represented by two aligned attackers on the king file. This is a direct FEN setup with board anchor 3k4/8/4p3/8/8/4R3/5Q2/6K1; no opening history is inferred from the diagram.
FEN: 3k4/8/4p3/8/8/4R3/5Q2/6K1 w - - 0 4
Question: What is the first concrete decision in this windmill position?
Answer cue: Windmill: preserve the alignment and calculate the defensive interposition before trading the front piece. Candidate line: Re4. The first candidate to test is Re4. Replay Re4 on board anchor 3k4/8/4p3/8/8/4R3/5Q2/6K1 and write down what changes after it.
Evidence boundary: Illustrative evidence for the stated mechanism; it is not a claim that the candidate move is forced or best in every position.
Reading the line
Read the first example (id example-1) as a complete record: continuation Kg7, source fide-laws. This is a direct FEN setup with board anchor 5k2/4p3/8/8/2Q5/8/4R3/6K1; no opening history is inferred from the diagram. If A does not expose two aligned attackers on the king file, correct the evidence before relying on the prose.
Read the second example (id example-2) separately: continuation Ra7, source fide-laws. Its feature is two aligned attackers on the king file; inventory side White; pieces Black king e8, Black pawn e7, White rook a6, White queen f2, White king g1; legal first moves Ra7, Ra8+, Rb6, Rc6, Rd6, Re6, Rf6, Rg6. Compare what changes between Kg7 and Ra7.
The defensive resource
The main mistake to avoid with Windmill is spotting a familiar motif but failing to test the defender's strongest legal resource. In A, the missing condition must be located in r1:5k2 | r2:4p3 | r3:8 | r4:8 | r5:2Q5 | r6:8 | r7:4R3 | r8:6K1 after Kg7; in B, test it against r1:4k3 | r2:4p3 | r3:R7 | r4:8 | r5:8 | r6:8 | r7:5Q2 | r8:6K1 after Ra7.
For Windmill, the first board's evidence boundary (after Kg7) is Illustrative evidence for the stated mechanism; it is not a claim that the candidate move is forced or best in every position. The second board's evidence boundary for Windmill is Illustrative evidence for the stated mechanism; it is not a claim that the candidate move is forced or best in every position. If a trusted source or careful calculation changes the conclusion around the final simplification, update both the evidence record and this article.
Practice from a real game
cover the answer, list checks and captures, and record the defender or escape square that decides whether the idea works. Start with A's side to move and r1:5k2 | r2:4p3 | r3:8 | r4:8 | r5:2Q5 | r6:8 | r7:4R3 | r8:6K1; record two candidates, the reply to Kg7 and the correction. Repeat with B's side to move and r1:4k3 | r2:4p3 | r3:R7 | r4:8 | r5:8 | r6:8 | r7:5Q2 | r8:6K1; the changed board tests transfer rather than diagram memory.
After the drill, compare your notes with Battery, Smothered Mate, Corridor Mate through a deflection from the critical square. Keep A's rejected alternative beside B's chosen line Ra7; this shows whether the conclusion survived a new board signature.
Sources and scope
Sources for Windmill: 1chess.online editorial scope, FIDE Laws of Chess. Source ids are original-analysis, fide-laws. Primary evidence hook: fide-laws. Comparisons and practice notes are editorial. Evidence revision: 2026-08-remediation-r2.
Scope note for board pair r1:5k2 | r2:4p3 | r3:8 | r4:8 | r5:2Q5 | r6:8 | r7:4R3 | r8:6K1 / r1:4k3 | r2:4p3 | r3:R7 | r4:8 | r5:8 | r6:8 | r7:5Q2 | r8:6K1: this page does not claim that Kg7 is an engine principal variation, that an exact endgame result is verified when the evidence says it is not, or that a tool output replaces human calculation. The conclusion is limited to the recorded boundary and the exception a counter-check that changes the calculation.