Many North American and African tonal languages undergo "syntagmatic displacement," as one tone is replaced by another in the event that the new tone is present elsewhere in the adjacent tones. Usually, these processes of assimilation occur from left to right. In some languages of West Africa, for example, an unaccented syllable takes the tone from the closest tone to its left. However, in East and Southeast Asia, "paradigmatic replacement" is a more common form of tone sandhi, as one tone changes to another in a certain environment, whether or not the new tone is already present in the surrounding words or morphemes.