The agent-trigger affixes are "-um-", "mag-", "man-", and "ma-". The difference between "mag-" and "-um-" is a source of confusion among learners of the language. Generally speaking, there are two main distinctions among many; "mag-" refers to externally directed actions and "-um-" for internally directed actions. For example, "bumilí" means "to buy" while "magbilí" means "to sell". However this isn't writ law for these affixes; there are exceptions for example, "mag-ahit" means "to shave oneself" while "umahit" means "to shave someone". "magbili" and "umahit" are rarely used; in southern dialects of Tagalog na- is used instead of -um-.