Building a Dichotomous Key

Every survey in this batch ends with the same problem: you have a plant or an animal in front of you and no idea what it is. Photographs and field guides work by recognition, which fails exactly when the thing is unfamiliar — which is when you needed help. A dichotomous key works the other way. It asks a series of either/or questions about features you can actually see, and each answer halves the possibilities. Carl Linnaeus and Jean-Baptiste Lamarck built the first ones in the eighteenth century, and the structure has not been improved on because it cannot be: a binary question is the most information a single observation can give. Building your own key for your own site is the fastest way to learn a local flora, and it produces something genuinely useful — a key to the twenty grasses in one meadow beats a national flora of nine hundred species for that meadow, every time.