A cryptarithm is a number puzzle in which a group of arithmetical operations has some or all of its digits replaced by letters or symbols, and where the original digits must be found. In such a puzzle, each letter or symbol represents a unique digit. The first example appeared in American Agriculturist in 1864. Specific types of cryptarithm include the alphametic, the digimetic, and the skeletal division. A digimetic is a cryptarithm in which digits are used to represent other digits.