A cage is the basic building-block of a Killer Sudoku puzzle. It is a set of contiguous cells, surrounded by a dotted outline or marked off with distinctive background color. The cage is marked (usually in the top left corner) with a number which represents the sum of all the cells in the cage.
The highlighted part of this grid shows a cage with 4 cells and a sum of 26.
Through application of the 45 rule, a solver may create additional split cages or hidden cages that subdivide or overlap the original given cages of a puzzle. These cages will only follow the Killer Convention if their cells see each other.