Hive
What is a gate, and when does it block a Beetle?
A gate is two adjacent pieces or stacks on either side of a path that leave no room for a piece to slide through between them. A Beetle is blocked only when both the space it leaves and the space it enters are lower than both stacks forming the gate. To test, first lift the moving Beetle off the board, since the piece itself does not count towards the heights. A single stack alone never forms a gate.