Concentric pencils, [50]

Cone of the second order, [59]

Conic, [73], [81]

Conjugate diameters, [114], [148]

Conjugate normal, [151]

Conjugate points and lines, [100], 109, [138], [139], [140]

Constants in an equation, [21]

Contingent relations, [180], [181]

Continuity, [180], [181]

Continuous correspondence, [9], [10], 21, [49]