The essential difference between horizontal and vertical policy-making is that in the former model, there is consultation between policy-makers at the same level, whereas in the latter model, policy is dictated from the top down. Horizontal policy-making is more democratic, and often more popular. However, it is also much slower. Various societies, such as the Roman Republic, have favored horizontal policy-making (debate in the Senate) in times of peace, but reverted to vertical policy-making (appointment of a temporary dictator) in wartime or in a crisis, when policies needed to be made and implemented quickly.
In practice, most policy-making is a mixture of horizontal and vertical elements. Occasionally, a president or CEO will decide on a policy alone and order everyone else to carry it out with no consultation. Conversely, in some communes or other small collective societies, every decision may be debated and voted on by the whole group. However, it is far more common for policies to begin with a small group at a high level, such as a committee in the Senate or the House of Representatives. These policies are debated horizontally before they are implemented vertically, and this may go on through several levels, with feedback as part of the process.