Rectangular six
A 2×3 rectangular six is the straight four's big brother: two centre points side by side, and the attacker can never occupy both. Whatever White tries, Black answers on the other centre point and the space falls into two eyes — the end points defend themselves. (Beware: the same shape sitting in the actual corner can be a ko, or judged dead outright by the bent-four-in-the-corner rule.) White to play: probe the shape and watch it live.