Quick answer
Underpromotion Tactic is a tactical idea: a promotion threat used as a tactic. Work from the first evidence position for Underpromotion Tactic, and begin by testing Bh7.
Begin from Bh7 in 6B1/8/7k/8/8/8/8/6K1 w - - 1 3. Start with the forcing geometry: checks, captures, threats, defenders and flight squares around the target. The evidence answer is: Underpromotion Tactic: calculate the promotion square and every checking or capturing resource before queening. Candidate line: Bh7.
In the Underpromotion Tactic example, notice: side White; pieces White bishop g8, Black king h6, White king g1; legal first moves Bh7, Bf7, Be6, Bd5, Bc4, Bb3, Ba2, Kf2. Do not export this inventory beyond the present example; it is a local observation, not a doctrine.
The tactical trigger
The first evidence board tests a promotion threat used as a tactic; the second board tests a promotion threat used as a tactic. For the pair of examples, install A from direct FEN setup and install B from direct FEN setup. Hub vocabulary: Chess Wiki · Tactics hub. Then compare Candidate Moves with Promotion Tactic, En Passant Tactic, Castling Tactic around the final simplification.
The two inventories make the distinction concrete. A contains side White; pieces White bishop g8, Black king h6, White king g1; legal first moves Bh7, Bf7, Be6, Bd5, Bc4, Bb3, Ba2, Kf2; B contains side Black; pieces Black king g8, White pawn g7, White king h1; legal first moves Kh7, Kxg7, Kf7. Compare the side to move before comparing the labels in the context of the last safe flight square.
Topic dossier for this page and board pair r1:6B1 | r2:8 | r3:7k | r4:8 | r5:8 | r6:8 | r7:8 | r8:6K1 / r1:6k1 | r2:6P1 | r3:8 | r4:8 | r5:8 | r6:8 | r7:8 | r8:7K: start with the final simplification; then test the last safe flight square; keep the overloaded attacker separate from the only defensive interposition; use the mating net's boundary as the transfer check; and reserve the exchange that removes a guard for the exception.
Calculation order
On the first board, calculate Bh7 after Bh7 and write down the opponent's best reply around the final simplification. On the second board, calculate Kh7 after Kh7 and check the last safe flight square. 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 exchange that removes a guard.
Key claim: Underpromotion Tactic is illustrated here by a promotion threat used as a tactic; the first concrete test is Bh7. The board anchor is 6B1/8/7k/8/8/8/8/6K1; 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 underpromotion tactic, calculate the winning idea, and test the defender's best resource?”, compare the recorded setup with Kh7 and the opponent's reply. The decision is useful only when the stated condition survives that concrete test on board 6k1/6P1/8/8/8/8/8/7K. 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 r1:6k1 | r2:6P1 | r3:8 | r4:8 | r5:8 | r6:8 | r7:8 | r8:7K. The false-positive test is whether the overloaded attacker appears without the only defensive interposition, so the conclusion is useful only when the stated condition survives that concrete test.
Evidence positions
example-1: a promotion threat used as a tactic
Show answer
Underpromotion Tactic: calculate the promotion square and every checking or capturing resource before queening. Candidate line: Bh7.
Why this position is here: Underpromotion Tactic is represented by a promotion threat used as a tactic. This is a direct FEN setup with board anchor 6B1/8/7k/8/8/8/8/6K1; no opening history is inferred from the diagram.
FEN: 6B1/8/7k/8/8/8/8/6K1 w - - 1 3
Question: What is the first concrete decision in this underpromotion tactic position?
Answer cue: Underpromotion Tactic: calculate the promotion square and every checking or capturing resource before queening. Candidate line: Bh7. The first candidate to test is Bh7. Replay Bh7 on board anchor 6B1/8/7k/8/8/8/8/6K1 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: a promotion threat used as a tactic
Show answer
Underpromotion Tactic: calculate the promotion square and every checking or capturing resource before queening. Candidate line: Kh7.
Why this position is here: Underpromotion Tactic is represented by a promotion threat used as a tactic. This is a direct FEN setup with board anchor 6k1/6P1/8/8/8/8/8/7K; no opening history is inferred from the diagram.
FEN: 6k1/6P1/8/8/8/8/8/7K b - - 2 2
Question: What is the first concrete decision in this underpromotion tactic position?
Answer cue: Underpromotion Tactic: calculate the promotion square and every checking or capturing resource before queening. Candidate line: Kh7. The first candidate to test is Kh7. Replay Kh7 on board anchor 6k1/6P1/8/8/8/8/8/7K 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: a promotion threat used as a tactic
Show answer
Underpromotion Tactic: calculate the promotion square and every checking or capturing resource before queening. Candidate line: Kh7.
Why this position is here: Underpromotion Tactic is represented by a promotion threat used as a tactic. This is a direct FEN setup with board anchor 7k/4N3/8/8/8/8/8/7K; no opening history is inferred from the diagram.
FEN: 7k/4N3/8/8/8/8/8/7K b - - 4 4
Question: What is the first concrete decision in this underpromotion tactic position?
Answer cue: Underpromotion Tactic: calculate the promotion square and every checking or capturing resource before queening. Candidate line: Kh7. The first candidate to test is Kh7. Replay Kh7 on board anchor 7k/4N3/8/8/8/8/8/7K 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 Bh7, source fide-laws. This is a direct FEN setup with board anchor 6B1/8/7k/8/8/8/8/6K1; no opening history is inferred from the diagram. If A does not expose a promotion threat used as a tactic, correct the evidence before relying on the prose.
Read the second example (id example-2) separately: continuation Kh7, source fide-laws. Its feature is a promotion threat used as a tactic; inventory side Black; pieces Black king g8, White pawn g7, White king h1; legal first moves Kh7, Kxg7, Kf7. Compare what changes between Bh7 and Kh7.
The defensive resource
The main mistake to avoid with Underpromotion Tactic is spotting a familiar motif but failing to test the defender's strongest legal resource. In A, the missing condition must be located in r1:6B1 | r2:8 | r3:7k | r4:8 | r5:8 | r6:8 | r7:8 | r8:6K1 after Bh7; in B, test it against r1:6k1 | r2:6P1 | r3:8 | r4:8 | r5:8 | r6:8 | r7:8 | r8:7K after Kh7.
For Underpromotion Tactic, the first board's evidence boundary (after Bh7) 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 Underpromotion Tactic 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 mating net's boundary, 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 r1:6B1 | r2:8 | r3:7k | r4:8 | r5:8 | r6:8 | r7:8 | r8:6K1; record two candidates, the reply to Bh7 and the correction. Repeat with B's side to move and r1:6k1 | r2:6P1 | r3:8 | r4:8 | r5:8 | r6:8 | r7:8 | r8:7K; the changed board tests transfer rather than diagram memory.
After the drill, compare your notes with Promotion Tactic, En Passant Tactic, Castling Tactic through the only defensive interposition. Keep A's rejected alternative beside B's chosen line Kh7; this shows whether the conclusion survived a new board signature.
Sources and scope
Sources for Underpromotion Tactic: 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 r1:6B1 | r2:8 | r3:7k | r4:8 | r5:8 | r6:8 | r7:8 | r8:6K1 / r1:6k1 | r2:6P1 | r3:8 | r4:8 | r5:8 | r6:8 | r7:8 | r8:7K: this page does not claim that Bh7 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 the overloaded attacker.