A 'Key Box' bet includes all possible combinations of selected 'key' horse(s) when combined with other 'non-key' horses in a box bet.

For Example, a $1 Trifecta Key Box: 1/2, 3, 4 This Key Box wager would cost $18. It covers eighteen possible combinations, 1/2, 3, 4/2, 3, 4 (6 combinations with the key horse finishing first) 2,3,4/1/2,3,4 (6 combinations with the key horse finishing second) 2,3,4/2,3,4/1 (6 combinations with the key horse finishing third) .

The term 'key box' means that you are keying a horse(s) with other selections in a box bet. Only your “key” horse is being used in all possible combinations.