1	Lip orifice a longitudinal fissure, length of lip; leaves 2, radical, stem a leafless scape.		5 Cypripedium acaule
+	Lip orifice basal, 4–37 mm; leaves 2–several, cauline.		(2)
2 (1)	Lip appearing angular, adaxial surface swollen near middle, apex abruptly deflected downward; lateral sepals distinct.		1 Cypripedium arietinum
+	Lip not angular, slender slipper-shaped to globose sac, apex not deflected; lateral sepals connate or nearly distinct.		(3)
3 (2)	Leaves 2, subopposite to alternate, near midpoint of stem (very rarely, leaves 3, closely alternate near midpoint of stem); lip prominently marked reddish or brownish between veins.		(4)
+	Leaves 3 or more (very rarely 2), alternate on basal to middle portion of stem or along length; lip yellow, ivory, or white, without extensive markings between veins.		(6)
4 (3)	Flowers (1–)2–4 in short nodding spike, dull yellow to reddish brown or dark purple, lip mostly 8–14(–25) mm.		2 Cypripedium fasciculatum
+	Flowers solitary, erect, white to yellowish or greenish, mottled with pink or brownish; lip 15–30 mm.		(5)
5 (4)	Petals slightly shorter than to equaling globose to obovate lip, constricted near apex, lanceolate-subpandurate in general outline; markings pink to magenta.		3 Cypripedium guttatum
+	Petals much shorter than oblance-fusiform to oblance-ovate lip, constricted near middle, acuminate-subpandurate to pandurate in general outline; markings brownish or tan.		4 Cypripedium yatabeanum
6 (3)	Petals flat, oblong to ovate, unmarked, white or green, yellow-green, or brownish.		(7)
+	Petals spirally twisted or undulate (rarely flat), linear-lanceolate, or rarely linear-oblong, spotted and striped (sometimes minutely so) or suffused with darker reddish brown or madder on yellowish or greenish ground color (rarely without markings).		(9)
7 (6)	Flowers large; lip 25–53 mm.		6 Cypripedium reginae
+	Flowers small; lip 11–20 mm.		(8)
8 (7)	Plants usually less than 35 cm; flowers 1(–2); lateral sepals nearly distinct to connate almost to apex.		7 Cypripedium passerinum
+	Plants usually much more than 35 cm; flowers 3–18(–22); lateral sepals connate almost to apex.		8 Cypripedium californicum
9 (6)	Lip orifice 27–37 mm; flowers very large; lip (41–)53–65 mm, ivory to pale yellow.		12 Cypripedium kentuckiense
+	Lip orifice usually less than 23 mm (rarely to 27 mm, then lip deep yellow); flowers smaller; lip mostly 15–45 mm (rarely to 54 mm, then lip deep yellow), white or yellow.		(10)
10 (9)	Lip white, rarely suffused with magenta; staminode lanceoloid or oblong-lanceoloid to ellipsoid.		(11)
+	Lip white or yellow; staminode cordiform-ovoid, deltoid to lance-ovoid, or ovoid-oblong (rarely staminode narrower or ellipsoid, then lip yellow).		(12)
11 (10)	Sepals and petals green or yellowish, usually spotted and striped with reddish brown or madder; dorsal sepal 15–35 mm; leaves lanceolate or elliptic to oblanceolate, ascending from insertions toward base or middle of stem.		9 Cypripedium candidum
+	Sepals and petals suffused with reddish brown or madder; dorsal sepal 33–60 mm; leaves suborbiculate to ovate or elliptic-lanceolate, ascending to spreading, inserted toward base or scattered along stem.		10 Cypripedium montanum
12 (10)	Lip white, 19–33 mm; sepals and petals suffused with reddish brown or madder, often intensely so (very rarely clear green), long and slender; dorsal sepal 33–60 mm, ratio of length to width 2.5–6.5; cordilleran.		10 Cypripedium montanum
+	Lip yellow (very rarely white), 15–54 mm; sepals and petals suffused with reddish brown or madder or more commonly spotted and striped (often minutely so), rarely unmarked, short and broad to long and slender; dorsal sepal 19–80 mm, ratio of length to width 1.3–3.7; widespread.		11 Cypripedium parviflorum