1 Leaves opposite (upper leaves sometimes subopposite or alternate); leaf lower surface glabrous, puberulent, or short-pilose, the pubescence not obscuring the green leaf surface; inflorescences (axillary) spikes, often leafy at their apices; herb to 1.5 m tall; [subgenus Duretia]

1 Plant a tree; petiole attachment to leaf blade peltate; leaves alternate; [tribe Cecropieae]

1 Plant an herb; petiole attachment to leaf blade marginal; leaves alternate or opposite.

..2 Leaves opposite.

....3 Plant with stinging trichomes, these having a distinct bulbous or cylindrical base, and a stiff, translucent apex; [tribe Urticeae]

....3 Plant without stinging trichomes (or these minute and not apparent), the non-stinging hairs (if present) soft and flexible, lacking a bulbous or cylindrical base.

......4 Flowers in axillary spikes; foliage dull, yellow-green; leaves 3-veined from the base, the 2 main side veins reaching the margin about 2/3s of the way from blade base to blade tip, the midvein with 1-2 or more prominent secondary veins borne near or past the midpoint and at a sharply acute angle to the midvein, these arching to the leaf margin; [tribe Boehmerieae]

......4 Flowers in axillary panicles or fascicles; foliage shiny, bright green; leaves 3-veined from the base, the 2 main side veins extending to the apex of the blade, the midvein with many secondary veins borne along its length at a nearly right angle, and connecting to the 2 main side veins rather than reaching the leaf margin; [tribe Lecantheae]

..2 Leaves alternate (at least above, if not throughout).

........5 Leaves toothed; plant either with or without stinging trichomes.

..........6 Flowers in terminal or axillary panicles; herb to 1.5 m tall, with stinging trichomes; leaf undersurfaces green; [tribe Urticeae]

........5 Leaves entire; plants without stinging trichomes.

............ 7 Stipules absent; [tribe Parietarieae]

............ 7 Stipules present; [tribe Boehmerieae]