1	Aerial stems lasting more than one year; main stem often unbranched; stomata sunken, in one row; strobili mucronate apically; sheath teeth membranous, deciduous, light brown or gray (E. subg. Hippochaete (Milde) Baker)		(2)
+	Aerial stems lasting one year or less; main stem often branched in regular whorls; stomata located on surfaces of aerial branches, in several rows; strobili blunt apically; sheath teeth leathery, persistent, blackish brown or reddish brown (E. subg. Equisetum)		(5)
2 (1)	Aerial stems irregularly undulate; main stem ca. 0.6 mm in diam., solid and without longitudinal canals in center, 7- or more ridged, with 3(-5) sheath teeth.		10 E. scirpoides
+	Aerial stems not irregularly undulate; main stem more than 1 mm in diam., hollow and with longitudinal canals in center, 6-ridged, with 6 or more sheath teeth		(3)
3 (2)	Mature main stem branched in whorls; sheath tubes grayish white or slightly reddish brown at apex.		7 E. ramosissimum
+	Mature main stem unbranched or rarely branched but not in whorls; sheath tubes blackish brown at apex		(4)
4 (3)	Main stem robust, 5-9 mm in diam. at middle, up to 1 m tall or taller; apices of sheath teeth falling off early; sheath teeth 2-ridged abaxially.		8 E. hyemale
+	Main stem slender, (1-)2-3(-4) mm in diam. at middle, 18-50 cm tall; sheath teeth persistent, 4-ridged abaxially.		9 E. variegatum
5 (1)	Aerial stems monomorphic, fertile stems same as sterile stems morphologically or young fertile stems slightly different from sterile branches		(6)
+	Aerial stems dimorphic, fertile stems without or with few shorter and finer whorled branches, obviously different from sterile branches		(8)
6 (5)	Plants large; aerial branches 40-60 cm, main stem 3-6 mm in diam. at middle; internodes of lower 1-3 nodes of main stems reddish brown, lustrous; main stem not branched in whorls or branched in whorls but branches much thinner than main stem.		3 E. fluviatile
+	Plants small; aerial branches 10-30 cm, main stem 1-2 mm in diam. at middle; internodes of lower 1-3 nodes of main stems blackish brown, not lustrous; main stem branched in whorls		(7)
7 (6)	Two sides of ridges of main stem and lateral branches with raised edges; upper main stem and lateral branches with 1 row of tubercles reaching apex of sheath teeth and a deep groove going through back of sheaths.		1 E. diffusum
+	Two sides of ridges of main stem and lateral branches rounded, with neither edges nor tubercles (but only laterally cut); groove shallow, going through back of sheaths.		2 E. palustre
8 (5)	Main sterile branch plus lateral branches narrower than 10 cm; whorled lateral branches of sterile branch pointing upward and forming a 30?or smaller angle with main stem; lateral branches approximately half of main stem in diam.; main stem branched or unbranched below middle; fertile branch unbranched.		6 E. arvense
+	Main sterile branch plus lateral branches up to 20 cm wide; whorled lateral branches of sterile branch pointing laterally or slightly upward and forming a 45?90?angle with main stem; lateral branches much thinner than half of main stem in diam.; main stem unbranched below middle; fertile branch eventually branched		(9)
9 (8)	Sheath teeth of sterile branch (2 or)3-5-lobed; lobes ovate-deltoid, reddish brown, two sides of ridges often with bristlelike protuberances; sheath teeth of lateral branches open.		4 E. sylvaticum
+	Sheath teeth of sterile branch 14-22-lobed; lobes narrowly deltoid, blackish brown at middle and light brown on margins, two sides of ridges often with tubercles; sheath teeth of lateral branches not open.		5 E. pratense