Descriptive geometry, [179]
Diameter, [107]
Directrix, [157], [158], [159], [160]
Double correspondence, [128], [130]
Double points of an involution, [124]
Double rays of an involution, [133], 134
Duality, [94], [104], [161], [180], [182]
Dupin (1784-1873), [174], [184]
Eccentricity of conic, [159]