A double six is a set of 12 lines in the 3-dimensional projective space, often written, the notation being due to Schlfli, as a matrix: and having the property that two lines among those 12 are secant if and only if they are not placed on the same line or column of the matrix.

In the game of dice, rolling double 2's is often called "Twos" or "Ducks," while rolling double 3's is sometimes called "Crabs." Double 4's are known as "Fours," double 5's as "Puppy Feet," and double 6's as "Boxcars" or "Midnight." These names can vary depending on regional or personal preferences.