The Mumonkan is, with the blue-cliff-record, the principal collection of Koan Practices in the Chan/Zen tradition. Each case presents a brief encounter between master and student, accompanied by Wumen’s verse and comment — often sharpening the koan rather than resolving it.
The first case is the most famous: a monk asks Zhaozhou whether a dog has buddha-nature; Zhaozhou answers mu (no/nothing). This “mu” became the first koan given to generations of Rinzai students — the gate into which concept cannot pass.