Insufficient material is an automatic draw that occurs when neither side has enough pieces to possibly deliver checkmate.
Some endings simply cannot be won no matter how badly one side plays. King versus king, king and bishop versus king, and king and knight versus king are all dead draws — there is no legal sequence that ends in checkmate.
The draw is automatic and immediate: the moment the last piece that could deliver mate is captured, the game ends as a draw without anyone needing to claim it. King and two knights versus a lone king is also a draw in the sense that mate cannot be forced, though it isn’t flagged automatically because mate is theoretically possible.
Knowing which material can’t mate saves you in lost positions: if you can trade down to king and a lone bishop or knight against the enemy king, you escape with half a point.
King vs king, king and bishop vs king, and king and knight vs king are automatic draws — and two same-coloured bishops can’t mate either. In each case checkmate is impossible, so the game ends drawn at once.
King and two knights cannot force mate against a lone king, so it is a practical draw. It isn’t declared automatically, though, because a checkmate position is technically possible if the defender blunders into it.
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