Dichotomous key

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A dichotomous key is a series of questions with only two alternatives. There are only two options at each step. The purpose is to make an identification or to reach a decision. The system has long been used for identifying animals and plants. However, in principle, it can be used for many other tasks, such as fault-finding in complex machinery.

Example of a diagnostic dichotomous key for some eastern United States oaks based on leaf characteristics

1. Leaves usually without teeth or lobes: 2
1. Leaves usually with teeth or lobes: 5
2. Leaves evergreen: 3
2. Leaves not evergreen: 4
3. Mature plant a large tree — Southern live oak Quercus virginiana
3. Mature plant a small shrub — Dwarf live oak Quercus minima
4. Leaf narrow, about 4-6 times as long as broad — Willow oak Quercus phellos
4. Leaf broad, about 2-3 times as long as broad — Shingle oak Quercus imbricaria
5. Lobes or teeth bristle-tipped: 6
5. Lobes or teeth rounded or blunt-pointed, no bristles: 7
6. Leaves mostly with 3 lobes — Blackjack oak Quercus marilandica
6. Leaves mostly with 7-9 lobes — Northern red oak Quercus rubra
7. Leaves with 5-9 deep lobes — White oak Quercus alba
7. Leaves with 21-27 shallow lobes — Swamp chestnut oak Quercus prinus

This key first differentiates between oaks with entire leaves with normally smooth margins (live oaks, Willow oak, Shingle oak), and other oaks with lobed or toothed leaves. The following steps created smaller and smaller groups (e. g., red oak, white oak), until the species has been keyed out.