In discussion surrounding another recent question, the fact came up that a "dropped" pawn in Shogi isn't allowed to deliver immediate checkmate, whereas any other type of piece can be dropped to deliver checkmate. This has gotten me (not a Shogi player at all) wondering why that distinction is in the rules:

Is there a particular rationale, perhaps coming from the other rules of Shogi, for singling out pawns as being unable to deliver checkmate via a drop?

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    I play Shogi. There's no reason I can think of for this rule. Perhaps it makes delivering mate too easy. Perhaps it's unseemly for a lowly pawn to kill a Shogun. – Tony Ennis Jul 20 '12 at 4:20
  • shouldn't this be considered off-topic? – prusswan Aug 29 '12 at 8:41
  • @prusswan: meta.chess.stackexchange.com/q/1/167 – ETD Aug 29 '12 at 16:20
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    @prusswan, Unlike Go, all three of chess, Shogi and Xiangqi trace their origins to the common ancestor game Chaturanga (en.wikipedia.org/wiki/Shogi#History). In that light I think it is reasonable to call them variants of one another. I see what you mean that someone could take offense to Shogi being called a chess variant specifically, but it goes that direction here just because chess is our primary focus. If this were "Shogi.stackexchange" instead, we'd call chess a Shogi variant. – ETD Aug 29 '12 at 18:18
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    @GrizzyRawrz, yes there is. When another shogi question was asked today, I added a [shogi] tag to it. And when I realized that that tag was apparently new, I decided to add the tag to this old question that I knew to exist. Even re-tags bump questions to the front (somewhat unfortunately). – ETD Apr 29 '14 at 4:14

I found this on the web:

The rule that a pawn can not give checkmate probably stems from the rule that a piece may not be placed in a position from where it is unable to promote or continue. A pawn may only give checkmate by being placed directly in front of the emperor; a position from where it is unable to move or promote. It can not take the emperor piece as it could with other pieces, and it can not be promoted until it moves again.

In Shogi, violating rules such as "Two pawns can not occupy the same column" or "a piece can not be placed in a position where it is unable to move or promote" results in an immediate loss for the one who violated the rule. Thus, by placing a pawn in this location, the player automatically loses the game by violation of a rule. The checkmate that the player gives doesn't matter because it was obtained through the rule's violation.

In certain Shogi variations, such as yari shogi, a pawn can be used to give checkmate.

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    Interesting; that makes a lot of sense in light of the rule that a piece cannot be dropped somewhere that it is unable to move or promote. But if that is the whole reason not to be able to drop a pawn for checkmate, then I'd think it should also be the case that a lance (whose movement is any number of squares forward) cannot be dropped directly in front of the enemy king to deliver checkmate, since it would be unable to move too. But that seems to be allowed, from what I can tell. – ETD Aug 21 '12 at 13:55
  • Yes I see a bug there ;) A lance is capable of promoting if it is on the last 3 lines so checkmating outside those last 3 lines should be illegal :D – Rinzwind Aug 21 '12 at 14:32
  • The small quibble about dropping lances aside, assuming no other answer comes along soon, I'll accept this one, since it does provide some rationale for the rule on the basis of the other rules of shogi, just as I'd hoped for. Thanks, Rinzwind. – ETD Aug 21 '12 at 19:20
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    There is a mis-statement of a rule here. In Shogi, you may not drop a piece to a square where it may never promote - not can't promote. It would not be hard to find examples where other pieces would be disallowed from checkmating using the same logic as the above. – Tony Ennis Aug 22 '12 at 11:59
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    @TonyEnnis, To clarify one matter for me: true or false? In a position such that moving a pawn to a particular square would deliver checkmate, it is legal to deliver checkmate by dropping a lance onto that square. It is my understanding that that is true, and that that somewhat undermines the rationale given in Rinzwind's answer, since a lance would also have no legal moves on said square. – ETD Aug 22 '12 at 13:33

As I wrote the comment on @Rinzwind's posting, I had an idea allowing me to expand on the 'too easy' comment I made some time ago:

Perhaps mate can't be delivered by pawn drop, but not because it's is too easy, but because a pawn-in-hand is too easy to acquire. It removes the challenge from the game. It's really common to have a pawn or two in-hand. Even if I don't, I could almost certainly get one easily, by 'sacrificing' a bishop, rook, or perhaps a lance. Easy.

Say you have a pawn defended by a gold. If I need a pawn to give mate, I take the pawn with a rook. If you recapture, you're mated because I have a pawn in hand. Thus you must take some defensive move. Now I gobble the gold with the rook.

Perhaps the 'get a pawn any way you can' tactic was just too common.

  • Perhaps a comparison with crazyhouse/chessgi or hostage chess could provide some evidence in this direction? – hkBst Apr 2 '18 at 9:14

One theory that I find reasonable is that the prohibition was originally unique to tsume problems, which makes them more fun, and later re-imported to normal shogi as well.

Another interesting one is a claim that it's implemented to prevent cheating of dropping the hidden 19th pawn to checkmate and immediately stir the pieces to blur it.

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