Most ergonomic keyboards fall into two categories: fully split which separates the board into two pieces, and unibody split, also known as an Alice design, which angles the keys outward at the bottom. When the keys are rotated outward or split into two halves, it allows for a wider spread between your elbows for a more relaxed typing position. Other ergonomic features, such as thumb clusters, center tenting and negative tilting are sometimes added to either type of board.
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There's a tradeoff: a lower capacity means you can skip more space during queries (you zoom in faster), but the tree has more nodes and uses more memory. A higher capacity means fewer nodes but each node requires checking more points linearly. As a starting point, capacities between 4 and 16 are reasonable defaults, though the best value depends on your data distribution and query patterns.