Ko shogi (or 'broad chess') is a large-board variant of shogi, or Japanese chess. The game dates back to the turn of the 18th century and is based on Xiangqi and go as well as Shogi. Credit for its invention has been given to Confucian scholar Ogyu Sorai.

The objective is to capture the opponent's commanding pieces: The General, plus, if present, the Governor; otherwise, the Banner or Middle troop.

Ko shogi has the most complex promotion rules of any shogi variant.

A player's promotion zone is the enemy camp, consisting of the six farthest ranks that comprise the opponent's territory at setup. If a piece moves within this promotion zone, including moves into, out of, or wholly within the zone, then that player may promote the piece at the end of the turn.

However, there are other ways of promoting than entering the enemy camp:

• A piece that captures a commanding piece (the General, Governor, Middle troop, or Banner) promotes on the spot.
• A step mover which captures a Sumo wrestler, Dragon ascending, Roaming assault, or Thunderclap, promotes on the spot.
• The Cavalryman promotes if it captures a European cannon.
• When the Clerk promotes to Master at arms, the Advance guards and Rear guards, promote as well, while no enemy poison flame.

There are other circumstances that may prevent or revert promotion:

• If the Taoist priest is captured, the Drum and Banner can no longer promote, and if either or both have already promoted, then they immediately revert to Drum or Banner.
• Whenever the Immaculate light is within 5 intersections of the Twelve-mile fog along one of the eight prime directions, the fog reverts back to a Taoist priest.

The following restrictions apply to capture:

• The Twelve-mile fog cannot be shot from within a distance of five intersections.
• The European cannon cannot shoot a Shield unit or an Imperial base. It can only shoot at a Twelve-mile fog that is at least six intersections away.
• The Long bow and Crossbow cannot shoot an Imperial base, Shield, Shield unit, Chariot, Chariot unit, Gun carriage, or Twelve-mile fog.
• The Cannon cannot shoot an Imperial base, Shield unit, or Twelve-mile fog.

See Also: https://en.wikipedia.org/wiki/Ko_shogi

Advanced version
Differences from the official version:

• Alternative setup
• The promotion zone increased
• The Pawns cannot step to backward
• The Pawns can step on two points to forward, if the Drum upgraded (Thunderclap or Banner and drums exists)
• The Rear Guard step on two points to forward
• The Engineers can upgrade the technical units (Chariot, Shield, Long bow, Cannon, European Cannon, Banner and Drum)