The Roma represent a highly heterogeneous group both linguistically and culturally- a fact often neglected or unknown by the surrounding populations. The most important factor which defines individual Romani subgroups is endogamy. This means that members of a certain group can only marry within the group. The subgroups are often divided to further groups which also practice endogamy. An example is the subgroup of Vlax Roma which is an umbrella term for various endogamous units.