In Biology. a single-access key (also calle "sequential key", "analytical key", or "pathway key") is an identification key where the sequence and structure of identification steps is fixed by the author of the key. At each point in the key multiple options are offered, each option leading to the next choice. The options are commonly called "leads", the set of leads at a given point a "couplet". 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"). The majority of single-access keys are dichotomous.