Every red box has the same structure: each input in the top half is compared to the corresponding input in the bottom half, with all arrows pointing down (dark red) or all up (light red). If the inputs happen to form a bitonic sequence (a single nondecreasing sequence followed by a single nonincreasing one or vice versa), then the output will form two bitonic sequences. The top half of the output will be bitonic, and the bottom half will be bitonic, with every element of the top half less than or equal to every element of the bottom half (for dark red) or vice versa (for light red). This theorem is not obvious, but can be verified by carefully considering all the cases of how the various inputs might compare, using the zero-one principle, where a bitonic sequence is a sequence of 0s and 1s that contains no more than two "10" or "01" subsequences.