Characters used to circumscribe fern families often relate to cryptic features, e.g., presence or absence of annulus and its orientation, length and diameter (number of cells) of the sporangial stalk, nature of the spores (whether all one kind or dissimilar on a given plant), spore shape, color, ornamentation, and number per sporangium, stem and petiole cross-section anatomy, and adaxial grooving (or lack thereof) of the rachis and costae. In addition, chromosome number, a trait almost never assessable by those doing identifications, is often considered in circumscription of families. As much as possible, these definitive characters have been avoided in the key below, which makes the key artificial in part.

In the flora area, species in Hymenophyllaceae, Grammitidaceae, and Vittariaceae sometimes exist primarily or entirely as independently reproducing populations of gametophytes. See species descriptions in those families.

Key

1	Leaves grasslike, blade not expanded; spore-bearing structures embedded in leaf bases or on short stalks 1-2 mm at leaf bases, at or below ground level; plants of aquatic, semiaquatic, or vernally wet habitats.	> 2
1	Leaves scalelike, needlelike, or with expanded blades, if grasslike then sporangia at tips of leaves; spore-bearing structures usually borne aboveground (except Marsilea), at or near leaf bases, or on blade margins or surfaces, occasionally on modified erect stalks or in hardened sporocarps; plants variously aquatic, terrestrial, or epiphytic.	> 3
2	Leaves tightly clustered on compact 2- or 3-lobed corms, rarely on short-creeping stems, not circinate; sporangia embedded singly in swollen leaf bases.	Isoëtaceae
2	Leaves borne on filiform, short-creeping stems, circinate; sporangia numerous in globose, hairy sporocarps attached by stalk 1-2 mm at bases of leaves.	Marsileaceae
3	Plants bearing inconspicuous scalelike or needlelike leaves (microphylls) less than 2 cm, or leaves longer and rushlike, all leaves with single unbranched vein; sporangia (synangia in Psilotum) occurring singly in leaf axils or aggregated in conelike structures, or pendent from peltate sporophylls in terminal cones.	> 4
3	Plants commonly bearing expanded leaves (megaphylls) with branched vasculature in blades, or leaves reduced to nonlaminate vascular tissue with terminal sporangium-bearing lobes in some Schizaeaceae, or leaves rounded to ovate, crowded, and floating in Azolla and Salvinia; sporangia borne in clusters (sori) of various shapes, hardened sporocarps, or covering blade surface.	> 7
4	Stems (and branches, if any) jointed, usually fluted, hollow, often rough from silica deposited in cells; leaves borne in whorls at each node, fused at base to form sheath but with free tips that may be caducous; sporangia aggregated into terminal strobili with polygonal segments.	Equisetaceae
4	Stems and branches, if any, not obviously jointed, not fluted or hollow, lacking silica; leaves spirally or oppositely arranged; sporangia variously arranged.	> 5
5	Leaves ca. 1-2 mm, scalelike, generally 5-30 cm apart, borne on nearly naked, repeatedly dichotomous stems; sporangia fused into 3-lobed clusters (synangia).	Psilotaceae
5	Leaves generally greater than 2 mm, closely placed and usually less than 5 mm apart, often less than 2 mm apart; sporangia separate, not fused into 3-lobed structures.	> 6
6	Sporangia borne singly in leaf axils, the leaves unmodified or modified and aggregated in cylindrical strobili mostly 3-25 mm wide at branch tips; spores of 1 size, less than 50 µm in diam.	Lycopodiaceae
6	Sporangia commonly borne in flattened or 4-sided strobili 1-2.5(-3.5) mm wide at branch tips (except Selaginella selaginoides with cylindrical cones 4-6 mm wide); spores of 2 sizes, large megaspores greater than 300 µm diam., borne singly or in groups to 4, and minute microspores in uncountable masses.	Selaginellaceae
7	Plants floating in water or sometimes rooted in mud at edges of ponds or streams, or in wet meadows and vernally inundated areas; plants less than 30(-50) cm.	> 8
7	Plants terrestrial, on rock, or epiphytic, if rooted in mud then leaves erect and more than 30 cm.	> 12
8	Sporangia borne on erect, nonlaminate stalks often equaling or exceeding sterile blades, fertile stalks arising at base of or below sterile blades; spores all 1 kind.	Ophioglossaceae
8	Sporangia borne in hardened sporocarps or on blade tissue, never on erect nonlaminate stalks; spores all 1 kind or of 2 kinds, 1 distinctly larger than the other.	> 9
9	Blades more than 10 cm, pinnately lobed or divided, dimorphic, fertile blades taller and with narrower segments; petioles commonly inflated; spores all 1 kind.	Parkeriaceae
9	Blades less than 10 cm, not pinnately lobed or divided, monomorphic; petioles filiform or absent; spores of 2 kinds borne in specialized sporocarps.	> 10
10	Photosynthetic blades 4-parted in cloverlike fashion on filiform petioles; leaves borne on short- to long-creeping stems usually rooted in mud.	Marsileaceae
10	Photosynthetic blades undivided, not cloverlike, entire, round to oval or ovate; leaves borne on usually floating stems.	> 11
11	Blades mostly 5-15 mm, round to oval, with conspicuous hairs adaxially.	Salviniaceae
11	Blades mostly less than 1 mm, ovate, papillate but lacking hairs adaxially.	Azollaceae
12	Sporangia fused laterally into 2-rowed, long-stalked linear units (synangia) and opening by double row of pores or slits; sterile portions of blades entire (except Cheiroglossa).	Ophioglossaceae
12	Sporangia discrete, not fused into synangia; sterile portions of blades often divided.	> 13
13	Leaves with rachis twining, high-climbing, pinna midribs dichotomously forked or pseudodichotomously forked (i.e., with dormant hairy bud at fork); sporangia borne on lateral lobes of pinnules, each sporangium subtended by indusiumlike flap.	Lygodiaceae
13	Leaves not twining, occasionally scandent, leaf branching various (stems hemiepiphytic and climbing in Maxonia); sporangia never both borne singly and subtended by indusiumlike flap.	> 14
14	Blades pseudodichotomously branched (i.e., at many forks with hairy dormant bud between branches), scrambling or trailing; sori exindusiate, sporangia commonly 4-8(-15) per sorus; spores more than 200 per sporangium.	Gleicheniaceae
14	Blades not pseudodichotomously branched (usually pinnate, palmate, or undivided), lacking dormant, hairy buds, rarely creeping (Hypolepis, Pteridium); sori exindusiate or indusiate, sporangia usually more than 15 per sorus; spore number various.	> 15
15	Sporangial capsules (excluding stalks) greater than 0.4 mm, opening by action of subterminal annulus or by thin-walled and poorly developed lateral patch, or annulus seemingly lacking; sporangia usually with several hundreds to thousands of spores; sporangia sessile or with very short stalk with 4 or more cells in cross section; sporangia usually borne on nonlaminate (not green) tissue or on naked or specialized stalks.	> 16
15	Sporangial capsules (excluding stalks) less than 0.4 mm, opening by action of vertical or slightly oblique thick-walled annulus, usually containing 16-64 spores; sporangia short- to long-stalked with 1-3 rows of stalk cells in cross section, rarely sessile; sporangia usually borne on blade tissue, rarely on nonlaminate tissue.	> 19
16	Leaves without expanded blades, grasslike; fertile blades terminated by pinnately branched or palmately arranged lobes (sporophores) that bear sporangia; annulus subapical.	Schizaeaceae
16	Leaves with expanded sterile blades, not grasslike; fertile blades not as above, if sporangia borne on specialized stalks or lobes then these not terminal; annulus subapical, lateral, or absent.	> 17
17	Sporangial capsules pear-shaped, ca. 0.5 mm, borne on 2 nonlaminate, branched, spikelike stalks (highly modified pair of basal pinnae), or leaves wholly dimorphic and less than 10 cm; annulus subapical and girdling sporangium.	Anemiaceae
17	Sporangial capsules globose, 0.5-1 mm diam., borne either on single nonlaminate, branched stalk attached below or at base of blade, or on specialized areas at middle or tip of leaf with otherwise "normal" blade, or leaves dimorphic and more than 50 cm; annulus not apical and not girdling sporangium.	> 18
18	Fertile portions of leaves inserted near base of sterile portion, long-stalked; sterile blades usually less than 30 cm; spores tranparent, thousands per sporangium.	Ophioglossaceae
18	Fertile portions of leaves apical or in middle of sterile blades, or fertile and sterile leaves dimorphic; sterile blades, or those with sterile portions, usually greater than 30 cm; spores green, hundreds per sporangium.	Osmundaceae
19	Blades 1-cell thick between veins, translucent, lacking stomates; sporangia borne within tubular, conic, or 2-valved marginal involucres (indusia); leaves minute to small, 0.5-20(-40) cm; scales absent at stem apex, hairs sometimes present.	Hymenophyllaceae
19	Blades 3-many cells thick between veins, usually opaque, stomates present; sporgangia borne otherwise; leaves small to large, usually greater than 20 cm; scales (rarely hairs) present at stem apices.	> 20
20	Plants hemiepiphytic, with stems rooted in ground and then climbing trees.	Dryopteridaceae
20	Plants terrestrial, on rock, or strictly epiphytic.	> 21
21	Blades linear, 1-3 mm wide, entire; sori linear in 2 submarginal grooves, 1 on each side of midrib, without indusium; stem scales strongly clathrate (latticelike, with dark lateral walls and thin, translucent surface walls); pendent epiphytes.	Vittariaceae
21	Blades usually pinnatifid or more divided, if simple then more than 5 mm wide; sori generally round or oblong; stem scales not clathrate (except Asplenium); terrestrial, if epiphytic, not pendent.	> 22
22	Sori elongate, in 1 row on each side of and immediately adjacent to costae (and costules in more divided blades), extending nearly entire length of pinnae or end-to-end (chainlike); indusia introrse, with openings facing costae (or costules) and away from margins.	Blechnaceae
22	Sori elongate to round, usually many per pinna, if elongate and parallel to costae then not immediately adjacent to them; indusia (when present) opening variously, commonly extrorse.	> 23
23	Stems and petiole bases bearing hairs 1 cell wide, lacking scales 2 or more cells wide; sori marginal or nearly so; indusia cuplike, attached proximally and sometimes along sides, or formed by revolute blade margins.	Dennstaedtiaceae
23	Stems and petiole bases bearing scales several cells wide; sori marginal or well back from margin, or sporangia covering blade surfaces; indusia linear, reniform, or peltate, rarely cuplike, or absent, sometimes replaced by false indusium formed by reflexed, recurved, or revolute blade margins.	> 24
24	Sori elongate along veins, never marginal, usually with linear indusium.	> 25
24	Sori variously shaped, often round, sometimes marginal, or sporangia covering surfaces, if elongate along veins then without indusium.	> 26
25	Scales on stems and petiole bases clathrate; sori generally along 1 side of vein only; sporangial stalks 1 cell thick; petioles with 2 back-to-back C-shaped vascular bundles in cross section, these fused into an X-shape distally.	Aspleniaceae
25	Scales on stems and petiole bases not clathrate; sori often along both sides of veins or curved around end of vein; sporangial stalks 2-3 cells thick; petiole vasculature various in cross section, if bundles 2 then these crescent-shaped and fused into a U-shape distally.	Dryopteridaceae
26	Blades pinnatifid, less than 5 cm ×5 mm; sporangial stalks 1 cell thick; spores trilete, green; [rare, known in flora area from single population of gametophytes and sterile sporophytes in North Carolina].	Grammitidaceae
26	Blades simple, pinnatifid, 1-pinnate, or more divided, commonly more than 5 cm ×5 mm; sporangial stalks 2-3 cells thick; spores trilete or monolete, rarely green.	> 27
27	Blades simple, pinnatifid, or pinnatisect nearly to rachis, rarely 1-pinnate, lobes (pinnae) entire or nearly so; leaves borne on short phyllopodia and cleanly abscising at this junction with age; sori without indusium; spores bilateral, colorless or yellowish, often transparent.	Polypodiaceae
27	Blades palmate, 1-pinnate, or commonly more divided, if pinnatisect, at least some of the lobes themselves pinnatifid; leaves not borne on phyllopodia, petioles continuous, not cleanly abscising with age; sori with or without indusium; spores bilateral to tetrahedral-globose, variously colored, usually opaque.	> 28
28	Sori marginal or submarginal, sometimes in continuous bands and/or covered by revolute or recurved margins of segments; spores globose-tetrahedral, trilete.	Pteridaceae
28	Sori medial on veins or covering surfaces of fertile segments, if submarginal, then not elongate or covered by revolute or recurved margins; spores various.	> 29
29	Sporangia completely covering surfaces of fertile segments.	> 30
29	Sporangia along veins or aggregated into discrete, round or oblong sori.	> 31
30	Stems short, petiole bases contiguous; blades variously dissected, if 1-pinnate then greater than 1 m; petioles not green-winged.	Pteridaceae
30	Stems long-creeping, petiole bases well separated; blades 1-pinnate, less than 0.5 m; petioles green-winged.	Dryopteridaceae
31	Costae, veins, and sometimes tissue between veins with hairs sparse to dense, needlelike or sometimes stellate, transparent, never obscuring abaxial leaf surface; blades 1-pinnate, 2-pinnatifid, or 1-pinnate-pinnatifid (2-3-pinnate in Macrothelypteris); petioles in cross section with 2 crescent-shaped vascular bundles at base, these united into a U-shape distally; spores bilateral, monolete.	Thelypteridaceae
31	Costae, veins, and tissue between veins lacking needlelike, transparent hairs, or (in Bommeria) hairs so dense as to obscure abaxial leaf surface, sometimes with brownish hairlike scales (proscales) or glandular trichomes; blades variously divided; petiole vasculature various, with 1-many vascular bundles; spores various.	> 32
32	Sori without indusium, sporangia extending along veins; leaves monomorphic; blades pentagonal or (in Pityrogramma) lanceolate; blades abaxially whitish to yellowish with powder or exudate (except Bommeria and 2 spp. of Argyrochosma); petioles with 1 (2 in Pityrogramma) vascular bundle; spores trilete.	Pteridaceae
32	Sori with indusium, or if without indusium then round (indusia concealed by recurved blade margins in Onoclea and Matteuccia, which have dimorphic, fertile and sterile, leaves); blades generally lanceolate or deltate, lacking powdery covering; petioles with 2-many vascular bundles; spores monolete.	Dryopteridaceae