There are many ways to divide people. It can be done by class, language, religion or in some other manner. These divisions are critical to many, if not all, modern democracies. The numerous different nations that are encompassed within a state can influence that state in many ways, among those ways is in the area of national politics.
These political divisions among people are called cleavages. Cleavages can be formally defined in a number of ways, to Stein Rokkan and Seymour Lipset (from the text book Representative Government in Modern Europe, pg. 235)for something to be a called a cleavage it must include three traits. The first necessary trait is to have a social division that distinctly separates people. It cannot simply be a political difference in ideology, like Republicans and Democrats in the United States. Secondly, those who are a part of this distinct background must be aware that they belong to the group and willing to act on it. Lastly, the distinct group must have some kind of organization. With these characteristics, people are able to influence the organizational structure and behavior of representative government. The most common type of cleavage is the center-periphery cleavage.
A center-periphery cleavage can be found in most countries. This cleavage involves the "center", who are people that are part of a culture that represents the "center" of politics; these people are those who are the majority in that country and are the ones in the leadership. The "periphery" is just the opposite; these are a people (or many different peoples) that share a culture that is politically in opposition to those who are in the center. The center wants to assimilate those who are part of the periphery to ease tensions that might arise from the competing sides. The periphery strives to preserve its culture and to be represented in the legislative body. The center at times feels the pressure of the periphery because of the independent ideology that comes from the group via its representatives in the legislative body.