Traditionally drawn straight onto the ground with chalk: a rectangle split into ten small squares for the "citizens", capped by two half-circles for the "mandarins". Children play it with pebbles; the large mandarin stones are worth ten citizens each when captured.
Variant implemented
Standard two-player Ô Ăn Quan: 10 citizen squares (5 stones each) plus two mandarin half-circles holding one large "quan" stone worth 10 points; bidirectional sowing; capture across a gap.
Board and pieces
A rectangle drawn on the ground, divided into ten small squares for the "citizens" (dân), with a half-circle at each end for the "mandarins" (quan).
Setup
Five stones in each citizen square. One large stone in each mandarin half-circle.
Turn
Take everything from one of your own five citizen squares and sow it, one stone per square, in a direction of your choosing — clockwise or anticlockwise. Then look at the square after the one your last stone fell in: if it is a citizen square holding stones, scoop it up and keep sowing.
Capture and scoring
If the square after your last stone is empty and the square beyond that holds stones, you capture that far square. Then check past it: if the pattern "empty, occupied" repeats, capture again, and keep chaining.
Win condition
The game ends when both mandarin squares are empty. Each player adds the stones still in their own squares to their score. A captured mandarin stone counts as ten. Highest score wins.
Draw condition
Equal scores are a draw.
Special rules
If all five of your squares are empty at the start of your turn you must "sow the fields": place one stone from your own captured pile into each of them. If you have fewer than five captured stones the game ends there.
Computer opponent
Alpha-beta over both sowing directions from every non-empty square. The evaluation weights captured stones and the proximity of an un-captured mandarin.