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The first player to start a stack in a range of 1, 2, or 3 spaces will "win" a stack battle in that range.

What does that mean? Say Black places a piece at b3. White places a piece at c5. Black adds to the stack at b3, White adds to the stack at c5. This happens again next turn, giving each player a 3-stack. Black can now unstack via either path to c5, taking control of the stack at c5. This will be true throughout the game. A better option for White is to start their stack 4 steps away. In this example, rather than c5 White might play at (as one option) c6. At the end of a hypothetical stack battle, both White and Black will have a 4-stack. Black cannot simply unstack over White, since the stack limit is 4.

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Covering your opponent's stacks is an important tactic. It can make the resulting stack more difficult to unstack (because you will leave their pieces behind), but it leaves your opponent with fewer options.