Cabalistic Chessboard

Cabalistic Square

Cabalistic Triangle

Calculator

Calendar

Caliban

Caligula 

Calligram

Camel

Cantor Set

Captain's Age

Capture

Cardan Rings

Cards

Cards Shuffle

Carmichael Number

Carousel Number

Carroll Wounded

Carroll Clock 

Carroll Diagram

Casting Out Nines

Catalan Equation 

Catalan Number 

Catalogues

Cell

Centered Number

Chameleon

Chang Formula

Checker

Checkerboard

Cheery Sequence

Cheskers

Chess

Chessboard

Chessboard Multiplication

Chessboard Number

Chifu-Chemulpo

Chinese Checkers

Chinese Jigsaw

Chinese Rings

Chinese Triangle

Christians and Turks

Chromatic Chessboard

Chromatic Number

Chromatic Square

Chuquet's Deniers 

Chuquet's Jewels 

Cipher

Ciphering

Circle

Circle Squaring

Circuit

Circular Differentiation

Circular Jump

Circular Permutation

Circular Tangram

Circus

Clock

Close-Packing

Code

Coin

Coincidence Ratio

Collatz Algorithm

Columbian Number

Column

Color

Colored Cube

Colored Hats

Colored Square

Colored Tower

Colored Triangle

Colors

Combination

Combinatorial Game 

Combinatorial Lattice

Combinatorial Recreation

Common Difference

Compartments Magic Square

Complementary Densities

Complementary Dominoes 

Complementary Magic Lattice 

Complementary Numbers

Composite Magic Square 

Composite Number

Computer

Comstock Formula

Concentric Magic Square 

Condition

Configuration

Congruent Figures

Congruent Numbers

Conjecture

Conjugated Elements

Connexion

Consecutive Numbers

Conservative Number

Construction

Contack

Continental Board

Contraction

Convergent Partition

Convict Man

Conway Arrow

Conway Kite

Cooked Recreation

Coprime

Correlative Figure

Correspondent Element

Counting

Course

Course Square

Cow-boys

Cow Knot

Crested Number

Cross  

Cross-Cap 

Cross-Number Puzzle

Cross-Pegs

Crossed Magic Square

Crossing (1)

Crossing (2)

Crossroad

Cryptarithm

Cryptarithmetic

Cryptarithmic Recreation

Cryptogram

Cryptography

Cube

Cube Duplication 

Cubic Number

Cuboctahedron

Cubosquare Identity

Curious Magic Square 

Curious Number  

Curry Square

Curry Rectangle

Curry Triangle

Curve

Curvilinear Hexagon

Curvilinear Lattice

Curvilinear Quadrilateral

Curvilinear Triangle

Cutting-out

Cycle

Cyclic Number

Cyclic Path

Cylinder

Cylinder's Principle