Complex sentence: If the entire key consists of exactly two choices at each branching point, the key is called dichotomous, else polytomous (or in false analogy: "polychotomous").
Split into simpler sentences: If the entire key consists of exactly two choices at each branching point, the key is called dichotomous. If not, it is described as polytomous (or in false analogy: "polychotomous").