Table 1.

Chronological overview of classification schemes employed for C-shaped roots and root canals

Structure	Method	Classification	Figure	Reference
Root	Transverse sectioning	Fused side: ++ = deep concave groove, + = shallow concave groove, ± = flat, − = convex; Unfused side: ℧ = inverted Ω, U = U-shaped, V = V-shaped, − = flat	3a	Fukuya (1976)
Root canal	Transverse sectioning	1 = three canals, 2 = C-shaped dotted canal, 3 = C-shaped lined canal, 4 = circle	3a	Fukuya (1976)
Root	Visual inspection	1 = deep and wide groove on lingual side with a concavity or groove on buccal side, both continuing to root apex; 2 = as type 1, but having sharp root apex; 3 = deep and wide groove on lingual side, continuing to root apex; 4 = deep and wide groove on lingual side, slightly C-shaped at root apex; 5 = deep groove on lingual side, slightly C-shaped at root apex; 6 = shallow groove on lingual side, continuing to apical area, sharp root apex; 7 = shallow groove on lingual side continues to less than 1/2 of root length, fused root apex; 8 = no groove, completely fused; 9 = shallow groove on lingual and buccal side, lingual groove deeper than buccal, fused root apex; 10 = deep groove on lingual and buccal side, fused root apex; 11 = shallow groove on lingual and buccal side, fused root apex; 12 = shallow groove on lingual side, deep and wide groove on buccal side continuing to root apex; 13 = shallow groove on lingual side, deep and wide groove on buccal side, sharp root apex; 14 = groove on buccal side, fused root apex	3b	Kotoku (1985)
Root	Transverse sectioning	1 = none of the grooves are extremely pronounced, 2 = one or two of the grooves are extremely pronounced, 3 = tooth with supernumerary radicular structure	–	Carlsen (1990)
Root canal	Transverse sectioning	1 = one primary main canal, 2A = one C-shaped main canal, 2B = two secondary main canals, 2C = three secondary main canals, 2AC = one secondary main canal and a combined canal, 3 = primary/secondary main canal and a supernumerary canal	3c	Carlsen (1990)
Root canal	Histology	I = continuously C-shaped canal (i.e., C-shaped canal outline without separation), II = semicolon-shaped canal, III = tooth has two or more discrete canals	–	Melton et al. (1991)
Root canal	Transverse sectioning	C1 = root canal in the form of an uninterrupted C, C2 = root canal semicolon-shaped, with angle α smaller and angle β larger than 60°, C3 = root canal in the form of separate canals, with angles α and β smaller than 60°, C4 = a single round or oval root canal, C5 = absence of a root canal lumen	4a–e	Fan et al. (2004a)
Root canal	X-ray imaging	I = canals merge into one major canal before exiting at the apical foramen, a partial dentine fusion area may appear in the coronal and/or middle portion of the canal system; II = separate mesial and distal canal located at the mesial distal part of the root, symmetry of the mesial and distal canal is present along the longitudinal axis of the root from a bucco-lingual view; III = separate mesial and distal canal are evident, distal canal may have a large isthmus across the furcation area from a bucco-lingual view, which commonly results in the mesial and distal canals being asymmetrical	6b	Fan et al. (2007)