Nichols algebras naturally appear in the theory of quantized enveloping algebras of Kac–Moody algebras and in the classification of finite-dimensional pointed Hopf algebras. Assuming that the base field has characteristic zero a list of finite-dimensional rank 2 Nichols algebras of diagonal type is given. Each of them is described in terms of generators and relations. Kharchenko's restricted Poincaré–Birkhoff–Witt basis is visualized using full binary trees.